McCain favours providing arms to rebel forces in Syria.
A State Department official said the department was aware of McCain crossing into Syrian territory Monday, but referred further questions to McCain's office. McCain spokeswoman Rachel Dean confirmed the Monday trip, but declined further comment.
The visit took place at the same time as meetings in Paris involving efforts to secure participation of Syria's fractured opposition in an international peace conference in Geneva.
And in Brussels, the European Union decided late Monday to lift the arms embargo on the Syrian opposition while maintaining all other sanctions against President Bashar Assad's regime after June 1, British Foreign Secretary William Hague said.
Two years of violence in Syria have killed more than 70,000 people, according to the United Nations. President Barack Obama has demanded that Assad leave power, while Russia has stood by Syria, its closest ally in the Arab world.
McCain, the 2008 Republican presidential candidate, has been a fierce critic of Obama administration policy in Syria while stopping short of backing U.S. ground troops in the country, but he supports aggressive military steps against the Assad regime.
Gen. Salem Idris, chief of the Supreme Military Council of the Free Syrian Army, accompanied McCain across the Turkey-Syria border. McCain met with leaders of the Free Syrian Army from across the country, who asked him for increased U.S. support, including supplying the rebels with heavy weapons, enforcing a no-fly zone, and conducting airstrikes on Syrian government and Hezbollah forces, according to The Daily Beast, which first reported the senator's visit.
Such unannounced trips to world hotspots by U.S. politicians are not common.
The White House declined to comment late Monday.
Last week, the Senate Foreign Relations Committee voted to provide weapons to the rebels in Syria, as well as military training to vetted rebel groups and sanctions against anyone who sells oil or transfers arms to the Assad regime. McCain is a member of the committee.
Associated Press writer Bradley Klapper in Paris contributed to this report.Suggest a correction