Three Americans were killed and another was wounded Wednesday when a remote-controlled bomb exploded under a U.S. diplomatic vehicle in the Gaza Strip (search), tearing apart an armored van.

It was the first attack on a U.S. target in three years of Israeli-Palestinian fighting and likely will cause the United States and its allies to pressure the Palestinian Authority (search) to take action against militant groups.

In a strongly worded statement, President Bush blamed Palestinian officials for the attack, which wounded another American. "Palestinian authorities should have acted long ago to fight terror in all its forms," Bush said.

The blast went off around 10:15 a.m. as a three-car U.S. diplomatic convoy with a Palestinian police escort drove near a gas station on the outskirts of Beit Lahiya in the northern Gaza Strip. The FBI is sending a team to investigate.

The three dead and the wounded man were American security personnel accompanying diplomats from the U.S. Embassy in Tel Aviv, said U.S. ambassador Dan Kurtzer. They were heading to interview Palestinian candidates for Fulbright scholarships at the education ministry.

The Palestinian driver of the vehicle also was wounded. The wounded American was initially treated at a Gaza hospital and was awaiting transfer to Soroka Hospital in the Israeli town of Beersheba. The bodies of the Americans were brought across the border to Israel to a forensic institute near Tel Aviv.

In response to the attack, the U.S. embassy in Tel Aviv advised all Americans to leave the Gaza Strip and asked the Israeli government to help evacuate Americans.

Between 200 to 400 Americans, some of them of Palestinian descent, work in the Gaza Strip, many for aid groups, Kurtzer said.

"The United States government recommends that all U.S. citizens depart the area as expeditiously as possible, while avoiding the area of the attack," the statement read. It also asked American citizens in the West Bank (search) to take precautions against possible further attacks.

Palestinian leader Yasser Arafat (search) condemned the attack, calling it an "awful crime" and said he ordered an investigation.

There was no immediate claim of responsibility for the attack, which occurred on the Palestinian side of a checkpoint near the border of Israel. Two of the most prominent militant groups that fight for Palestinian independence -- Islamic Jihad (search) and Hamas (search) -- both denied involvement and said they don't attack Americans.

Israel later announced orders to expel three Palestinian militants from the West Bank to the Gaza Strip. The decision came after it issued similar orders a day earlier against 15 other Palestinians -- raising criticism from the Palestinians and human rights groups.

'We Saw Bodies'

The blast gouged a deep crater into an unpaved stretch of road. The attack tore the van in half and flipped it over, leaving the wreckage twisted with the tires up in the air. The pavement was stained with blood and littered with bits of flesh that were collected by Palestinian paramedics.

State Department spokeswoman Brooke Summers said the blast came from a "previously planted explosive device."

An AP reporter saw a gray wire with an on-off switch leading from the scene of the attack to a small concrete room at the side of the road.

Mohammed Radwan, a Palestinian taxi driver, was at the gas station when the blast went off as the third U.S. car passed.

"The first two cars drove quickly and stopped far form the explosion," Radwan said. "Palestinian security people jumped out of the car and rushed to the car that had blown up. When I tried to approach them, they shouted at me to leave. I saw two people covered with blood lying next to the car."

Avi Isacharoff was a witness to the carnage.

"We saw bodies, parts of bodies, we saw parts of cars about 30 meters from the explosion itself," Isacharoff told Fox News. The car "wasn't even split in half, it was just upside down, only the shield was left in it ... I really couldn't believe it was a car or American car … just the windshield was left, body parts were all over… it was just a huge mess."

When U.S. investigators arrived to photograph the scene, about a dozen Palestinian youths pelted them with rocks and they cut their stay short. Palestinian police beat some people in the crowd while pushing the spectators back.

U.S. convoys travel in Gaza almost daily, and usually bear diplomatic license plates. Most take the same route on the main north-south road in the strip as where Wednesday's blast occurred.

Another Roadblock for Peace

Wednesday's attack could deal a major blow to Palestinian efforts to bring more international monitors to the region.

"This is an act that was acted against the Palestinian people, against the national interest of the Palestinian people, these were people that were coming to help us … rebuild schools and infrastructures," Saeb Erekat, a Palestinian cabinet member, told Fox News.

"This is very unfortunate, very tragic and stand assured that no effort will be spared in order to investigate the matter … this will not be tolerated under any circumstances."

Erekat said his government wants to get back to the negotiating table to hammer out differences in the U.S.-backed Mideast "roadmap" for peace.

Israeli government spokesman Ranann Gissin told Fox News that Arafat's Palestinian Authority is to blame.

"All of the excuses stop today," Gissin said. "This incident took place in an area under full control of the Palestinian authorities ... [you must] "draw the conclusion about the involvement and culpability of the Palestinian Authority."

Palestinian militants have attacked Israeli army and settler convoys in Gaza repeatedly in the past three years of fighting.

"We strongly condemn this incident and we will conduct an investigation and we will follow it to find the source of this attack," Palestinian Prime Minister Ahmed Qureia said.

Qureia called Secretary of State Colin Powell to express his condolences and promise swift action.

A State Department official told Fox News that while U.S. officials are glad to see immediate denunciations of the attack, Powell's message to the Palestinians is blunt: what are you going to do about it? The official said Washington expects the Palestinians to turn over any and all information about who launched this attack.

Israeli and U.S. officials said the attack underscored the need to dismantle Palestinian militant groups -- a requirement of the stalled peace plan.

The focus recently has been on trying to get all the Palestinian security forces -- some of which are controlled by Arafat -- under Qureia's control to carry out a crackdown. Arafat has resisted.

"The failure to create effective Palestinian security forces dedicated to fighting terror continues to cost lives," Bush said in a statement. "There must be an empowered prime minister who controls all Palestinian forces -- reforms that continue to be blocked by Yasser Arafat."

Attacks on U.S. targets have taken place in other Arab countries, including Lebanon, Saudi Arabia and now Iraq. In October last year, an American administrator for the U.S. Agency for International Development was gunned down in Amman, Jordan, in an assassination thought linked to Al Qaeda.

But in the bloody conflict between Israel and the Palestinians, there has been an unofficial policy of "hands-off" the Americans -- though 45 Americans have been killed in homicide bombings or by Israeli soldiers taking action against militants.

Fox News' Jennifer Griffin, Teri Schultz and The Associated Press contributed to this report.