DUBAI, United Arab Emirates — The U.S. Embassy in the Emirates closed Wednesday after a "specific threat" against it. Anti-American protests and fears of a terror attack prompted tighter security at other potential U.S. targets in the Mideast.

The U.S. Embassy in Riyadh, Saudi Arabia, briefly closed Wednesday 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.

Security was heightened after Israel's assassination of Sheik Ahmed Yassin, founder of the Hamas militant group in the Gaza Strip, prompted calls for revenge against Israeli and American targets. 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.

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.

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.

The State Department has issued a new worldwide advisory for Americans overseas, saying they face increasing threats from terror groups such as Hamas and al-Qaida.

But 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.

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.

German President Johannes Rau, who was touring Africa, canceled a planned stop in Djibouti. Rau planned to visit German naval troops patrolling the Indian Ocean off Djibouti as part of the U.S.-led war on terror.

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.

U.S. President George 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."

Bush told reporters in Washington, "We take every threat seriously in this administration."

On Tuesday, the State Department also issued travel advisories for the Middle East and North Africa, Israel and Iraq, saying credible information indicated terrorist groups may be planning attacks against American targets in the Middle East in response to Yassin's death.

The department urged vigilance in the Persian Gulf, Arabian Peninsula, Red Sea and North Africa.

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.

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."

On Wednesday, the United States lifted a warning for Americans to avoid nonessential travel to Turkey issued after November suicide attacks in Istanbul that were blamed on al-Qaida.

The decision lifting the warning came after a "reassessment of the security situation," a U.S. official said.

The truck bombings targeted two synagogues, the British Consulate, and a London-based bank, killing 62 people.