The warning, issued in an updated travel advisory, called the current situation in Israel "volatile and extremely dangerous."
"Israeli authorities are concerned about a possible increase in suicide bombings in Israel," the State Department said, adding: "The U.S. Government has received information indicating that American interests within Israel could be the focus of terrorist attacks.
"For that reason, American citizens are cautioned that a greater danger may exist in the vicinity of restaurants, businesses, and other places associated with U.S. interests and/or located near U.S. official buildings, such as the U.S. Embassy in Tel Aviv and the U.S. Consulate General in Jerusalem."
The warning also extended to American citizens working or taking part in other activities in Gaza and the West Bank.
"Violence between the Israeli Defense Forces (IDF) and militant groups based in the Gaza Strip and the West Bank can flare up without notice," the advisory said.
Also, foreigners — including Americans — who have been working with pro-Palestinian groups in Gaza should be careful. Some were "assaulted and injured in the Occupied Territories by Israeli settlers and harassed by the IDF.
"Those taking part in demonstrations, non-violent resistance, and 'direct action,' are advised to cease such activity for their own safety."
This likely refers to a group known as the International Solidarity Movement, said political science professor Nathan Brown, a senior associate with the Carnegie Endowment for International Peace in Washington.
Brown, whose specialty is in Arab and Palestinian politics, said International Solidarity Movement is non-violent and it marches to raise international awareness to Palestinian issues, often in hot-spot areas like the West Bank and Gaza. Members of the group are mainly American and European.
"The Israelis basically regard them as an unfriendly cause in the territories," Brown said, referring to Gaza and the West Bank. Under the current situation, being considered unfriendly is not a welcome status, he said.
Huwaida Arraf, co-founder and spokeswoman for the group, estimated about two dozen U.S. citizens who are members of her group are currently protesting in the West Bank.
Arraf, who also is an international law student at American University in Washington, said she doesn't plan on heeding the State Department's warning, and doesn't expect other members to do so either.
"We would be, in effect, turning our back on grave human rights abuses," Arraf said.
According to the State Department warning, an American also was captured in the West Bank in June and held captive several hours by armed militants. The reference appears to concern a Brown University student who was visiting the West Bank city of Nablus.
Efforts to reach the student also were not successful, and a call to the State Department public affairs staff was not immediately returned.
The travel warning also says that U.S. government employees are prohibited from traveling to the West Bank and Gaza unless they are on "mission-critical business," and they are prohibited from using public transportation.
Brown said he returned two weeks ago from the West Bank, and says now is not the time for the uninitiated to make their first trip to the region.
"Everything is hostage to the political situation, and the political situation now is awful. So it makes travel difficult for an experienced traveler," Brown said.
To complicate matters, Brown said that places like Tel Aviv, and to some degree, Jerusalem, are putting on a business-as-usual face regarding the conflicts in Gaza and the Lebanese border.
"You wouldn't even know that there's a crisis ... It really kind of varies from place to place," he said.
The northern areas, which contain tourist sites, are a different story. Locals are evacuating and even the experienced travelers are wary, Brown said.
"This is certainly not a time to go to the north as a tourist," Brown said. "It's essentially on a war footing."
The complete advisory follows:
Israel, the West Bank and Gaza Travel Warning
July 19, 2006
This Travel Warning is being issued to update information on the general security environment in Israel, Jerusalem, the West Bank, and the Gaza Strip, and to reiterate threats to American citizens and U.S. interests in those respective locations. Terrorist aggression from Lebanon and numerous rocket attacks into Israel have resulted in fatalities and injuries to civilians. This is a volatile and extremely dangerous situation. In addition to this, the conduct of the Palestinian Authority government led by Hamas and actions such as the kidnapping of an Israeli soldier by Hamas have caused increased instability in the West Bank and the Gaza Strip. Violent demonstrations and armed conflicts between supporters of the Hamas and Fatah factions have increased in the Gaza Strip and spread to the West Bank. Overall conditions of lawlessness prevail in the Gaza Strip, Israeli military operations continue, and areas of violent conflict shift rapidly and unpredictably. This warning supersedes the Travel Warning issued February 27, 2006.
In light of this situation, the Department of State urges U.S. citizens to weigh carefully the risk of travel to Israel and Jerusalem at this time and to defer travel to affected parts of Israel (missile attacks have focused on the area north of an east-west line from the coastal city of Haifa to Tiberius on the western shore of the Sea of Galilee, including those cities, and on Sderot and Ashkelon in the south). In addition, the Department of State urges U.S. citizens to defer travel to the West Bank and to avoid all travel to the Gaza Strip. American citizens in the Gaza Strip should depart immediately, a recommendation the State Department has maintained since the deadly roadside bombing of a U.S. Embassy convoy in Gaza on October 15, 2003. U.S. government personnel are currently prohibited from all travel in Gaza. Militants have abducted Western citizens and held them for short periods, and terrorist organizations have threatened attacks against U.S. interests.
Violence between the Israeli Defense Forces (IDF) and militant groups based in the Gaza Strip and the West Bank can flare up without notice in the form of rocket attacks into Israel by Palestinian militants, targeted assassinations of Palestinians in the Gaza Strip by Israeli forces, and ongoing IDF security operations in the West Bank. The uncertain security conditions within the West Bank and Gaza continue as well, with the potential for violent protests, kidnappings, including of foreign nationals, and fighting between various armed factions. Daily inter-Palestinian violence in the Gaza Strip has spread to the West Bank. An American citizen was kidnapped and held captive for several hours by armed militants in the West Bank in June 2006.
In recent months, citizens of Western nations, including Americans, involved in pro-Palestinian volunteer efforts were assaulted and injured in the Occupied Territories by Israeli settlers and harassed by the IDF. Those taking part in demonstrations, non-violent resistance, and "direct action," are advised to cease such activity for their own safety.
For safety and security reasons, U.S. Government American personnel and dependents are prohibited from traveling to any cities, towns or settlements in the West Bank, except for mission-essential business or other approved purposes. For limited, personal travel, U.S. government personnel and family members are permitted to travel through the West Bank, using only Routes 1 and 90, to reach the Allenby/King Hussein Bridge or the Dead Sea coast near Ein Gedi and Masada. Each such transit requires prior notification to the Consulate General's security office and must occur during daylight hours. U.S. Government personnel and family members are permitted personal travel on Route 443 between Modi'in and Jerusalem during daylight hours only. Travel to the Gaza Strip by U.S. Government personnel is prohibited. Under policy guidance issued by the Secretary of State, exceptions to the prohibition on Gaza travel are only for official, mission-critical travel. Private American citizens also should avoid travel to these areas.
All travelers who enter or travel in the Gaza Strip or the West Bank should exercise particular care when approaching and traveling through checkpoints and should expect delays and difficulties. Travelers should also be aware they might not be allowed passage through checkpoints. Israeli authorities are concerned about a possible increase in suicide bombings in Israel. The January 19 suicide bombing in Tel Aviv, the December 5 suicide bombing in Netanya and a similar incident in Hadera in October are reminders of the precarious security environment. The U.S. Government has received information indicating that American interests within Israel could be the focus of terrorist attacks. For that reason, American citizens are cautioned that a greater danger may exist in the vicinity of restaurants, businesses, and other places associated with U.S. interests and/or located near U.S. official buildings, such as the U.S. Embassy in Tel Aviv and the U.S. Consulate General in Jerusalem.
Violent confrontation in Israel between organized criminal elements in public places have involved the use of bombs, grenades, anti-tank missiles, and small arms fire, and have taken place in cities frequented by tourists. In the past several years, some of these incidents have led to the death and injury of innocent bystanders.
The State Department urges American citizens to remain vigilant while traveling anywhere in Jerusalem, especially within the commercial and downtown areas of West Jerusalem. Israeli security services report that they continue to receive information of planned terrorist attacks in and around Jerusalem. In addition, American citizens should stay away from demonstrations and generally avoid crowded public places, such as restaurants and cafes, shopping and market areas and malls, pedestrian zones, public transportation of all kinds, including buses and trains and their respective stations/terminals, and the areas around them. Spontaneous or planned protests within the Old City are possible, especially after Friday prayers. Some of these protests have led to violent clashes.
U.S. government personnel are prohibited from using all public transportation. The Old City of Jerusalem is off-limits to them after dark during the entire week and between the hours of 11:00 A.M. and 2:00 P.M. on Fridays. U.S. government employees are also forbidden from patronizing discos and nightclubs.
Any American Citizen who intends to travel to Israel, Jerusalem, the West Bank or the Gaza Strip in spite of this and prior warnings should carefully review the Consular Information Sheet for Israel, the West Bank and the Gaza Strip. That reference describes other potential dangers and difficulties and offers detailed security recommendations. Palestinian-Americans face many additional obstacles and regulations that are described in that document.
Americans who remain in Israel, Jerusalem, the West Bank and the Gaza Strip are strongly encouraged to register with the Consular Section of the U.S. Embassy in Tel Aviv or the Consular Section of U.S. Consulate General in Jerusalem through the State Department's travel registration website, U.S. citizens who require emergency services may telephone the Consulate General in Jerusalem at (972) (2) 622-7250 or the Embassy in Tel Aviv at (972) (3) 519-7355.
FOXNews.com's Greg Simmons contributed to this report.