A TOW missile system on a Humvee is seen at Fort Pickett, Va., on Sept. 12, 2012.

Chris Stone/U.S. Marine Corps

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WASHINGTON — Syria's opposition fighters have been supplied with U.S.-made anti-tank missiles, the first time a major American weapons system has appeared in rebel hands.

It is unclear how the rebels obtained the wire-guided missiles, which are capable of penetrating heavy armor and fortifications and are standard in the U.S. military arsenal. The United States has sold them in the past to Turkey, among other countries, and the Pentagon approved the sale of 15,000 of the weapons to Saudi Arabia in December. Both countries aid Syrian opposition groups.

U.S. officials declined to discuss the origin of the weapons but did not dispute that the rebels have them.

Their appearance on the ground in Syria coincides with a U.S. commitment this year to escalate a CIA-run program to supply and train vetted "moderate" rebel groups, and to improve coordination with other opposition backers.

"The United States is committed to building the capacity of the moderate opposition, including through the provision of assistance to vetted members of the moderate armed opposition," said National Security Council spokeswoman Bernadette Meehan. "As we have said, we are not going to detail every single type of our assistance."

Videos showing rebels using the weapons were first uploaded to YouTube between April 1 and 5 by Harakat Hazm, a moderate insurgent splinter group, according to Charles Lister, a visiting fellow at the Brookings Doha Center, who was among the first to identify the so-called TOW ("Tube-launched, Optically-tracked, Wire-guided") anti-tank missiles.

In an article published last week by Jane's Defense Weekly, Lister noted that any country that transferred U.S. weapons to third parties was required to notify the United States and receive its approval.

The United States secretly supplied hundreds of TOWs to Iran during the Reagan administration's arms-for-hostages arrangement in the 1980s. But Lister noted that the weapons seen in the newly released videos appear in good condition and with configurations different from the 1980s version.

Iran is a principal backer of Syrian President Bashar al-Assad's government, the other side in the civil war.

While some dispute the difference the weapons can make in the Syrian conflict, now well into its third year, the appearance of the missiles caused a flurry of excitement among experts who closely chronicle weapons used by either side.

The rebel movement has become deeply splintered over the past year, with an influx of foreign fighters and al-Qaida-linked groups gaining prominence and increasingly fighting against more moderate opposition groups as well as against Syrian government forces.

The presence of Islamic radicals within the opposition has long made the Obama administration reluctant to supply weapons, a reticence that last year caused other prominent rebel backers, most notable among them Saudi Arabia, to complain about a lack of U.S. leadership.

Those complaints were heightened last fall, when President Barack Obama first approved, then stopped, a U.S. airstrike against Syrian targets associated with the government's chemical weapons program. Obama pulled back after Congress indicated it would not approve the strikes, and he subsequently negotiated an agreement with Russia — Assad's leading weapons supplier — to destroy Syria's chemical arsenal.

After many delays, the removal of Syria's chemical weapons and components is close to being on schedule for completion by the end of April. Destruction is to occur at sea by the end of June.

But even as Assad's weapons of mass destruction are being eliminated, government forces, using so-called "barrel bombs" and other weapons primarily targeting civilians, have regained some territory previously lost to the rebels.

Rebel losses, infighting and surging extremist groups caused the administration and its allies in Europe and the Persian Gulf to reassess their strategy early this year. In a meeting of intelligence chiefs in Washington, they agreed to increase and better coordinate their aid. Countries such as Qatar, which the Treasury Department alleged were funding extremists, agreed to stop.

According to a senior Treasury official, it is not yet clear how effective those agreements have been.

But officials from a broad array of partner governments have said that the United States is slowly expanding its role as a leader and as a supplier. Some disagreements remain, including over a Saudi proposal — opposed by the administration — to supply the rebels with shoulder-launched surface-to-air missiles capable of shooting down Syrian aircraft.