The U.S. embassy in Syria sent out a warning to Americans living in the area Monday to be very alert for expected retaliation from terrorist groups following the assassination Hamas (search) spiritual leader Sheik Ahmed Yassin (search).

The following warning was distributed by the U.S. Embassy in Damascus to all local American citizens:

"On Monday, March 22, Sheikh Ahmed Yassin, the founder and spiritual leader of Hamas, was killed by an Israeli air strike as he was leaving a Gaza City mosque. We expect that emotions and tensions will be extremely high over the next few days given his prominent role and following. Because of the potential for demonstrations, the embassy cautions U.S. citizens to avoid large gatherings and neighborhoods where anti-American sentiment could run high in the near term.
U.S. citizens are reminded that demonstrations in many parts of the world may have an anti-American character. Even demonstrations intended to be peaceful can turn into confrontational situations and possibly escalate into violence. U.S. citizens are encouraged to maintain a high level of vigilance and to take appropriate steps to increase their security awareness."

Palestinian militant groups are vowing revenge after Yassin was killed in an Israeli airstrike early Monday.

Hamas has even warned they would retaliate with threats not only against Israel but against the United States, as well.

The Lebanese guerrilla group Hezbollah (search) fired at Israeli army positions on the Lebanese border on Monday, hours after Yassin's assassination, security sources said.

Yassin was the most prominent Palestinian leader killed by Israel in more than three years of fighting, and his assassination was seen as a major escalation.

More than 200,000 Palestinians, some carrying billowing green Hamas flags, flooded the streets for the funeral procession, the largest gathering in Gaza City in recent memory. Thousands also took to the streets in the West Bank.

Fox News' Teri Schultz and The Associated Press contribute to this report.