Updated at 5:40 p.m. ET : Secretary of State Hillary Rodham Clinton, National Intelligence Director James Clapper and other top diplomatic and security officials will huddle this week with lawmakers for a closed-door meeting on growing anti-U.S. violence in the Middle East and northern Africa, officials told NBC News on Tuesday.
The classified briefing was put together for House members after al-Qaida in the Maghreb, the North African branch of the terrorist group, published a call for followers to launch attacks on U.S. embassies and to kill U.S. diplomats.
The statement appeared to have been published Saturday, but it didn't come to widespread Western attention until Tuesday, when the Middle East monitoring service IntelCenter alerted its clients to the threat's appearance on a militant website. It called for attacks on U.S. interests around the world, but especially in Algeria, Morocco, Tunisia and Mauritania.
The statement called the assassination last week of Christopher Stevens, the U.S. ambassador to Libya, a "gift" that would "bring the Americans to the path of salvation and stop their war against Muslims."
Stevens was killed in a raid on the U.S. Consulate in Benghazi, Libya, on Sept. 11, along with three consulate staff members.
Clinton didn't mention the briefing in remarks to reporters in Mexico City, where she is holding talks with Mexican leaders on drug-interdiction strategies, but she said: "We are taking aggressive steps to protect our people and our consulates and embassies around the world.
"We are reviewing our security posture at every post and working with host governments to be sure they know what our security needs are wherever necessary," she said. "I think that it is important at this moment for leaders to put themselves on the right side of this debate — to speak out clearly and unequivocally against violence, whoever incites it or conducts it "
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The rise in violence has coincided with anger in the Muslim world after the publication on YouTube of a short trailer for an unreleased movie called "Innocence of Muslims," which depicts the Prophet Muhammad as a gay, wife-beating child abuser. At least 28 deaths — including those of Stevens and the three other Americans last week — have been attributed to riots and violence in at least 20 countries in reaction to the video.
In Afghanistan, NATO forces enacted tighter security measures Tuesday after rioters attacked police on a road to the U.S. Embassy in Kabul and a suicide bomber blew up a bus near the Kabul airport, killing 12 foreign workers in an attack that Islamist militants said was in retaliation for the blasphemous video.
Col. Thomas Collins, a spokesman for the International Security Assistance Force, the NATO-led contingent overseeing security in Afghanistan, told NBC News that the measures would put a temporary halt to joint operations with Afghan forces unless they were approved by a regional commander at the level of a general.
"We did a very thorough assessment," Collins said. "We looked at where we are right now with this video being out and some heightened tensions.
"We just thought it would be smartest on a temporary basis to reduce the amount of exposure of our troops in certain areas," he said.
More than 50 international troops have been killed this year in so-called green-on-blue attacks carried out by Afghan forces or militants disguised in Afghan uniforms.
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