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About 100 U.S. Special Operations Forces have been ordered to evacuate Yemen because of a dramatic increase in sectarian violence, sources told NBC News on Saturday.

The U.S. commandos, including Green Berets and Navy Seals, have been training Yemeni military forces in counterterrorism operations, but the Americans have not been involved in direct ground combat maneuvers against militants.

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The move comes as al Qaeda fighters captured the capital of a southern Yemen province late Friday, leading to the deaths of about 20 soldiers, Reuters reported. Earlier, four suicide bombers hit a pair of crowded mosques in the capital of Sanaa, killing at least 137 people and injuring more than 300 others, officials said.

The American forces in Yemen have also been gathering intelligence to target al Qaeda-linked terrorists and other militants for U.S. airstrikes in the region.

For instance, an American military Predator drone launched two Hellfire missiles more than a week ago at a vehicle carrying a top leader of the al Shabaab terrorist network in southern Somalia. Adnan Garaar, the mastermind of the 2013 Westgate Mall terrorist attack in Nairobi, Kenya, died of his wounds several days later.

Sources told NBC News that the U.S. Special Operations Forces have been based in remote areas of Yemen and considered relatively secure from enemy threats. But one source said that with this week's deadly uptick in sectarian violence, evacuating American commandos makes sense: "The threat is too high," the source said. "Why take the risk?"

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— Jim Miklaszewski