A senior Obama administration official says a recent visit by the commander of Iran's elite Quds force to Damascus is the strongest sign yet that Iran is supplying weapons to aid Bashar Assad's crackdown on the Syrian people.

The U.S. has long said Iran is helping drive the deadly crackdown on dissent in Syria. But the official says the visit by Quds Force Commander Ghassem Soleimani provides a concrete example of direct high-level cooperation between Iran and Syria.

The official spoke on the condition of anonymity because the person was not authorized to speak publicly.

Iran is one of the few international allies that Syria has left after its nearly year-long crackdown.