Washington warned anti-American violence could increase as outrage inflamed by a film mocking the Prophet Muhammad spread to 11 countries in the Middle East. \"The risk of violence could increase both at home and abroad as the film continues to gain attention,\" the U.S. Department of Homeland Security and FBI said in a joint intelligence bulletin. \"Additionally, we judge that violent extremist groups in the United States could exploit anger over the film to advance their recruitment efforts,\" the bulletin said. Security at U.S. diplomatic offices worldwide was increased. Aircraft stood ready on runways in key cities to evacuate personnel. In Yemen, hundreds of young men scaled a U.S. Embassy security wall in Sanaa Thursday as several thousand protested outside. One young man yelled, \"Troops will not stand in our way in defending the honor of our prophet,\" The Wall Street Journal reported. Four protesters died in the clashes with security forces, Yemeni security officials told CNN. U.S. President Barack Obama thanked Yemeni President Abdu Rabbo Mansour Hadi over the phone \"for his swift condemnation\" of the violence and for immediately investigating the violence, the White House said. Secretary of State Hillary Clinton denounced both the anti-Islam video and the violence in Libya that took the life of U.S. Ambassador Christopher Stevens and three other Americans Tuesday night. She called the video \"reprehensible\" and said \"we absolutely reject its content and message,\" making clear the U.S. government had no role in it. At the same time, there was \"no justification -- none at all -- for responding to this video with violence,\" she said. The Libyan government said Thursday it arrested four people in connection with the deadly U.S. Consulate attack as Libyan and U.S. officials mounted a manhunt for others believed to be involved. The third and fourth Americans killed in the attack were identified by the State Department Thursday as security personnel Tyrone S. Woods and Glen A. Doherty, both former Navy SEALs. Earlier identified, in addition to Stevens, was State Department information officer Sean Smith. In Egypt, site of one of the largest protests, more than 200 people were injured in a third day of clashes outside the U.S. Embassy in Cairo Thursday, medical officials said. Obama Wednesday warned Egyptian President Mohamed Morsi U.S.-Egyptian relations would be jeopardized if Egyptian authorities failed to publicly and strongly condemn the anti-American attacks. Obama later told U.S. Spanish-language TV network Telemundo Egypt was not necessarily a U.S. \"ally.\" But the White House and State Department quickly played down the remark. \"The president, in diplomatic and legal terms, was speaking correctly, that we do not have an alliance treaty with Egypt,\" White House spokesman Jay Carney said Thursday, adding, \"\'Ally\' is a legal term of art.\" State Department spokeswoman Victoria Nuland, when asked repeatedly if the State Department still considered Egypt a major non-NATO ally -- a designation it was awarded in 1989 -- said simply, \"Yes.\" Egypt\'s Muslim Brotherhood called for a nationwide peaceful demonstration Friday to condemn the \"Innocence of Muslims\" video, which was produced in the United States. Other anti-American protests flared outside U.S. embassies in Tunisia and Morocco and in Tel Aviv, Israel. In Gaza, Palestinians demonstrated outside U.N. headquarters. Some 500 Iranians protested near the Swiss Embassy in Tehran, chanting \"Death to America!\" and calling for the film director\'s death. Switzerland represents U.S. interests in Iran because Washington and Tehran have no diplomatic relations. Several hundred protesters in Baghdad\'s predominantly Shiite Sadr City area marched through the streets chanting, \"America is the enemy of the people,\" while some people burned the American flag, CNN reported.