The assault appeared to be a copy-cat of the protest Tuesday night at the U.S Embassy in Cairo, when angry youths climbed the walls and brought down the flag, though they largely refrained from any material damage.
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" produced by anti-Islam campaigners in the U.S. that mocked Islam's Prophet Muhammad. The amateurish video was produced in the U.S. and excerpted on YouTube. It depicts Muhammad as a fraud, a womanizer and a madman in an overtly ridiculing way, showing him having sex and calling for massacres.
Egyptian protesters clashed Thursday with police near the U.S. Embassy in Cairo. 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."
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.
Associated Press writers Slobodan Lekic in Brussels and Lara Jakes in Baghdad contributed to this report.
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