The Embassy in the United Arab Emirates closed after a "specific threat" against it. The U.S. Embassy in Riyadh, Saudi Arabia, briefly closed after rumors spread of an explosion. Security was tightened even further near the embassy in Cairo, Egypt, and protesters were chased away from the embassy in Bahrain.
The alert comes two days after Israel's assassination of Sheik Ahmed Yassin, founder of Hamas, prompted calls for revenge against Israeli and American targets.
But on Wednesday, Hamas' new leader in Gaza, Abdel Aziz Rantisi, pulled back from threats against the United States, saying his group's militant activities are aimed solely at Israel.
The State Department issued a new worldwide advisory for Americans overseas, saying they face increasing threats from terror groups such as Hamas and al Qaeda.
"Credible information has indicated terrorist groups may be planning attacks against U.S. interests in the Middle East," a separate warning for the Middle East and North Africa read.
The worldwide caution said Washington is "deeply concerned about the heightened threat of terrorist attacks against U.S. citizens and interests abroad," as well as violent anti-American demonstrations.
It warned citizens "to maintain a high level of vigilance and to take appropriate steps to increase their security awareness."
Demonstrations against the United States have been held daily in the streets, on university campuses and in Palestinian refugee camps around the Middle East, attracting thousands of people.
On Wednesday, about 1,500 Cairo University students demonstrated after midday prayers, urging Hamas' militant wing to exact revenge for Yassin's killing. One demonstrator, Hamed Abdel Raouf, said they want Arab leaders to end all cooperation with Israel and politically and financially back Palestinian resistance movements.
At the American University in Cairo, about 500 students protested Yassin's killing.
In the tiny Gulf island country of Bahrain, more than 200 teenage boys skipped school and pelted the high walls surrounding the U.S. Embassy compound with stones, shouting, "Death to America and Israel!", before scattering to elude riot police using tear gas.
There were no casualties and no arrests were made.
"America is the root of all the problems, and we want this embassy in Bahrain to be shut down," said a teenage boy who identified himself only as Mohsen, a student from a nearby school.
Bahrain, like Egypt, is a close ally of the United States and home base to the U.S. Navy's 5th Fleet. It is one of the more Westernized countries in the Gulf, though anti-American sentiment has been rising.
Cdr. James Graybeal, 5th Fleet spokesman, said he cannot comment on security procedures taken, but "we take our security very seriously and we have appropriate security measures in place."
Hilary Olsin-Windecker, spokeswoman for the U.S. Embassy in Abu Dhabi, said the "specific threat" was received Wednesday morning. She declined further details.
The security situation will be reviewed over the Thursday-Friday weekend in the Emirates — when U.S. offices regularly are closed — to determine whether the building will reopen Saturday.
Olsin-Windecker said there were no specific threats against Americans in the United Arab Emirates and she was unaware of any threats to other U.S. embassies in the region.
President Bush expressed concern Tuesday about the Hamas threat against the United States in the wake of Yassin's killing in an Israeli helicopter missile strike. The group said America's backing of Israel made the assassination possible and "all the Muslims of the world will be honored to join in on the retaliation for this crime."
Mr. Bush told reporters in Washington, "We take every threat seriously in this administration."
The United States, while expressing concern about the killing, is widely viewed in the region as Israel's protector and as unwilling to curb its actions against Palestinians. According to Pentagon records, the United States provided more military aid to Israel from 1950 to 2002 — $51 billion — than to any other country.
A warning addressed to Americans in Israel and the occupied territories advised them to "depart Gaza as soon as it is safe to do so and to defer travel to Israel, the West Bank and Gaza due to current safety and security concerns."
According to the Israel warning, the risk is not just from terrorists.
"Americans have been killed, seriously injured, detained, and deported as a result of encounters with Israeli Defense Forces operations in Gaza and the West Bank," it warned.
Israel also took security precautions Wednesday, with a Foreign Ministry spokesman in Jerusalem saying it withdrew diplomats and their families from the Arab countries of Mauritania and Qatar. The families — two or three in each of the small missions — were brought home a week before a planned home leave for the Jewish holiday of Passover, Yonatan Peled said.
Other countries were similarly wary. German President Johannes Rau, who was touring Africa, canceled a planned stop in Djibouti. His office said he canceled on the advice of German intelligence agencies, who believed there was a credible threat by Islamic militants to try to assassinate him.