US Senator John McCain has quietly slipped into Syria for a meeting with rebels.
Spokeswoman Rachael Dean confirmed the Republican made the visit but declined further comment about the trip.
The visit took place amid meetings in Paris involving efforts to secure participation of Syria's fractured opposition in an international peace conference in Geneva.
Two years of violence in Syria has killed more than 70,000 people. President Barack Obama has demanded that Syrian President Bashar Assad leave power while Russia has stood by Syria, its closest ally in the Arab world.
Mr McCain has been a leading proponent of arming the rebels and other aggressive military steps against the Assad regime.
He has criticised Obama administration policy there while stopping short of backing US ground troops in Syria.