The Obama administration said it knew in advance of Republican senator John McCain's trip to Syria, a visit that raised anew the question of whether the United States will intervene in a civil war that has raged for over two years.
But US officials had little to say about the trip by McCain, an outspoken advocate for US military aid to the rebels fighting President Bashar al-Assad.
President Barack Obama has repeatedly shied away from US involvement in the conflict, which has claimed 80,000 lives, although his administration has sent food and medical supplies to Assad's opponents.
"We were aware, of course, that Senator McCain was going to make this trip," White House spokesman Jay Carney said. "And we look forward to speaking with Senator McCain upon his return to learn more about the trip."
McCain's office said his visit was organised by the Syrian Emergency Task Force (SETF), a US-based non-profit group that backs the Syrian opposition. He crossed into northern Syria from the border with Turkey on Monday and stayed several hours.
McCain saw General Salim Idris, leader of the Supreme Military Council of the Free Syrian Army coalition of rebel groups, as well as 18 rebel commanders.
"General Idris and FSA commanders asked that the United States increase its aid to the Free Syrian Army in the form of heavy weapons, a no-fly zone, and air strikes on Hezbollah," the SETF website said, referring to the Shiite militant group in Lebanon that backs Assad.
US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov have been trying to organise an international peace conference on Syria.
But Russia said that the European Union's failure to renew an arms embargo on Syria would undermine peace talks.