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American missions have been attacked in three Arab nations - Yemen, Egypt and Libya - that have faced persistent unrest and are struggling to restore law and order after last year's revolts deposed their authoritarian regimes.

Protesters smashed windows as they breached the embassy perimeter and reached the compound grounds, although they did not enter the main building housing the offices. Angry young men brought down the U.S. flag in the courtyard, burned it and replaced it with a black banner bearing Islam's declaration of faith - "There is no God but Allah."

Yemeni security forces who rushed to the scene fired in the air and used tear gas to disperse the demonstrators, driving them out of the compound after about 45 minutes and sealing off the surrounding streets. It was not immediately clear whether anyone was inside the embassy at the time of the attack.

Demonstrators removed the embassy's sign on the outer wall, set tires ablaze and pelted the compound with rocks.

The Yemeni Embassy in Washington condemned the attack and vowed to ensure the safety of foreign diplomats and to step up security measures around their missions in the country.

It was similar to an attack on the U.S. Embassy in the Egyptian capital on Tuesday night. A mob of Libyans also attacked the U.S. consulate in the eastern city of Benghazi on Tuesday, killing American Ambassador Chris Stevens and three other Americans.

Yemen is home to al-Qaida's most active branch and the United States is the main foreign supporter of the Yemeni government's counterterrorism campaign. The government on Tuesday announced that al-Qaida's No. 2 leader in Yemen was killed in an apparent U.S. airstrike, a major blow to the terror network.

The spreading violence comes as outrage grows over a movie called "Innocence of Muslims" that mocked Islam's Prophet Muhammad. The amateurish video was produced in the U.S. and excerpted on YouTube.

U.S. officials also were investigating whether the rampage in Libya was actually planned to coincide with the anniversary of the 9/11 terror attacks.

Egyptian protesters clashed Thursday with police near the U.S. Embassy in Cairo for the third day in a row. Police used tear gas to disperse the protesters and the two sides pelted each other with rocks. But unlike Tuesday, the police kept the protesters away from the embassy's compound.

The Interior Ministry, which is in charge of police, said 16 protesters and 13 policemen were wounded in the clashes, which broke out overnight and were ongoing. Twelve protesters have been arrested, it said.

Egyptian President Mohammed Morsi, speaking while on a visit to Brussels, vowed on Thursday not to allow attacks on foreign embassies in Cairo, saying the Egyptian people reject such "unlawful acts."

In Iraq on Thursday, hundreds of Shiite followers of the anti-American cleric Muqtada al-Sadr demanded the closure of the U.S. Embassy in Baghdad because of the film. Protesters burned American flags and carried banners reading, "We reject the attack on the Prophet Muhammad.

"No, no, to Israel! No, no to America!" thousands shouted in the Shiite stronghold of Sadr City in northeast Baghdad. "`Yes, yes for Messenger of God."

Afghanistan's government, meanwhile, sought to avert any protests as past anger over perceived insults to Islam has triggered violence in the country.

President Hamid Karzai canceled an official visit to Norway and spoke by phone with U.S. President Barack Obama to convey his condolences for the deaths of the U.S. ambassador to Libya and three other diplomats, a statement said. He also discussed the "film and the insulting of holy Islamic values," but the statement provided no other details.

Stevens was the first U.S. ambassador killed in the line of duty in 30 years.

The movie, "Innocence of Muslims," came to attention in Egypt after its trailer was dubbed into Arabic and posted on YouTube. The video-sharing website blocked access to it Wednesday. The trailer depicts Muhammad as a fraud, a womanizer and a madman in an overtly ridiculing way, showing him having sex and calling for massacres.

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