A senior Israeli military official said on Wednesday that Hezbollah militants fighting in Syria were using American-made armored personnel carriers that were originally supplied to the Lebanese Army.
Hezbollah, the Lebanese Shiite militia and political party, is fighting in Syria on the side of the Russian- and Iranian-backed government of President Bashar al-Assad against rebels seeking his ouster. Both the United States and Israel consider Hezbollah a terrorist organization.
If the A.P.C.s passed to Hezbollah from the United States-assisted Lebanese Army, that might point to a broader leakage of weapons to hostile groups, and to cooperation between the Lebanese armed forces and Hezbollah.
It would not be the first time that weapons supplied to a friendly Arab government by the United States ended up in the hands of militants. Shiite militias in Iraq have been photographed with American-made weapons. And this year, weapons given to Jordan by the United States and Saudi Arabia for Syrian rebels were stolen by Jordanian intelligence agents and ended up on the black market.
But it would be an odd turn if weapons supplied by the United States ended up in the hands of militants fighting for Mr. Assad, as the Obama administration has resisted giving some advanced weapons to the Syrian leader’s foes for fear that they would make their way into extremists’ possession.
Hezbollah is widely recognized as the most formidable military force in Lebanon, which the United States has tried to balance with support for Lebanon’s armed forces. While the two forces have historically remained separate, the escalation of the war in Syria has led to cooperation between them to secure Lebanon’s borders, according to Lebanese analysts close to the military.
The Israeli military official, who spoke on the condition of anonymity under army rules, showed a photograph of a number of military vehicles, including what he said were the American-supplied A.P.C.s, during an intelligence briefing for reporters at military headquarters in Tel Aviv. He said that Israel had shared this information with American officials a few weeks ago.
The official did not say how many of the A.P.C.s might have fallen into Hezbollah’s hands or when or how it had happened, but he said he believed they might have been “part of a deal” between the Lebanese Army and Hezbollah.
John Kirby, the State Department spokesman, said on Wednesday: “When this allegation was raised in November, the Department of Defense did a structural analysis of the armored personnel carriers in question at that time and concluded that these vehicles were not from the Lebanese armed forces. Our assessment remains the same now.”
The Defense Department says that a small number of the vehicles are in the possession of Hezbollah, but this has been the case for several years, Gordon Trowbridge, the deputy Pentagon press secretary, said Wednesday. The vehicles, he added, could have come from several sources in the region.
“Our assessment is that they didn’t come from the Lebanese armed forces,” Mr. Trowbridge said.
The State Department said last month that it was investigating whether Hezbollah was using American military equipment after grainy images posted on social media appeared to show Hezbollah displaying American-made armored vehicles at a military parade in Qusayr, in western Syria, near the border with Lebanon.
News outlets reported that the display included American-manufactured M113 armored personnel carriers of the type provided by the United States to the Lebanese Army. A State Department spokeswoman, Elizabeth Trudeau, said at the time that the United States would be “gravely concerned” if that were the case.
The Lebanese Army command issued a statement denying that the vehicles had been taken from its arsenal, the Lebanese news media reported, and M113 A.P.C.s are not uncommon in the region. The American Embassy in Lebanon did not respond to a request for comment.
But the Israeli military official said that Israel recognized the A.P.C.s as being of the specific type given to the Lebanese military by the United States.
The official added that Hezbollah was strengthening its grip on Lebanese state institutions and that some Lebanese Army security chiefs were close to the Hezbollah leadership. Hezbollah, the official said, was carrying out combined patrols with the Lebanese Army along the border with Israel and had determined the location and height of new watchtowers the Lebanese Army built along the border to monitor Israeli movements.
The Lebanese Army recently received Cessna aircraft and Hellfire missiles from the United States. The Israeli official said he believed the Lebanese armed forces would be “smart enough” not to hand those to Hezbollah.
Israel fought a monthlong war against Hezbollah, which is committed to destroying Israel, in 2006. It still views Hezbollah as a formidable enemy on its border and watches the group carefully.
Israel has declared a position of neutrality and nonintervention in the Syrian war, but American officials say it has bombed a number of convoys inside Syria that were carrying weapons for Hezbollah. Israel has not confirmed or denied its role in specific attacks, but says it acts to prevent the transfer of sophisticated weapons.
Hezbollah has about 8,000 fighters in Syria, the Israeli official said. It has lost about 1,700 fighters in Syria and thousands more have been injured, he added.
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