The Pentagon is unable to account for more than $500 million in U.S. military aid given to Yemen, amid fears that the weaponry, aircraft and equipment is at risk of being seized by Iranian-backed rebels or al-Qaeda, according to U.S. officials.

With Yemen in turmoil and its government splintering, the Defense Department has lost its ability to monitor the whereabouts of small arms, ammunition, night-vision goggles, patrol boats, vehicles and other supplies donated by the United States. The situation has grown worse since the United States closed its embassy in Sanaa, the capital, last month and withdrew many of its military advisers.

In recent weeks, members of Congress have held closed-door meetings with U.S. military officials to press for an accounting of the arms and equipment. Pentagon officials have said that they have little information to go on and that there is little they can do at this point to prevent the weapons and gear from falling into the wrong hands.

“We have to assume it’s completely compromised and gone,” said a legislative aide on Capitol Hill who spoke on the condition of anonymity because of the sensitivity of the matter.

U.S. military officials declined to comment for the record. A defense official, speaking on the condition of anonymity under ground rules set by the Pentagon, said there was no hard evidence that U.S. arms or equipment had been looted or confiscated. But the official acknowledged that the Pentagon had lost track of the items.

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