Last month, footage emerged of Hezbollah fighters operating M113 armored personnel carriers in Syria, where the militia -- blacklisted by Washington as a terrorist organization -- is fighting in support of Bashar al-Assad's regime.
Yesterday, a senior Israeli military official speaking on condition of anonymity said Israel believes these vehicles were drawn from stocks supplied by Washington to the Lebanese Armed Forces.
But, in Washington, State Department spokesman John Kirby said US officials have investigated and do not believe that Lebanon has violated its agreement not to transfer on US-supplied equipment.
"As we noted when this first came up, the Lebanese Armed Forces stated publicly that the vehicles depicted online were never part of their equipment roster," he added.
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"The LAF fully complies with end use monitoring requirements, continues to have an exemplary track record with US equipment and remains a valued partner in the fight against ISIL and other extremists," he said, using an acronym for the Islamic State group.
Pentagon spokesman Gordon Trowbridge told AFP that Hezbollah does indeed have a "small number" of M113 armored personnel carriers in its inventory.
"They've had them for a number of years," Trowbridge said. "They could have come from a variety of sources because it's a relatively common vehicle in the region."
Neither US spokesman said where Hezbollah's M113s might have come from if not from the Lebanese army.
Earlier, the anonymous Israeli military official had told reporters that Israeli intelligence had "recognised these specific APCs... As those given by the US to Lebanon".
He said new information had been shared with the United States "a few weeks ago" but did not specify how many armored personnel carriers were involved.
Israel fought a devastating war with the Lebanese Shiite militia Hezbollah in 2006, and closely monitors the group's activities.
More recently, the Iranian-funded Lebanese Shiite movement has been fighting alongside Assad's forces in Syria's civil war.