And in Brussels, the European Union decided late Monday to lift the arms embargo on the Syrian opposition while maintaining all other sanctions against Bashar Assad's regime after June 1, British Foreign Secretary William Hague said following the meeting.
Two years of violence in Syria has killed more than 70,000 people. President Barack Obama has demanded that Assad leave power, while Russia has stood by Syria, its closest ally in the Arab world.
McCain has been a fierce critic of Obama administration policy there while stopping short of backing U.S. ground troops in Syria, 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 heavy weapons, a no-fly zone and airstrikes on Syrian government and Hezbollah forces, according to The Daily Beast, which first reported the senator's unannounced visit.
The White House declined to comment late Monday.
A State Department official said the department was aware of McCain crossing into Syrian territory on Monday. Further questions were referred to McCain's office.
Last Tuesday, the Senate Foreign Relations Committee voted to provide weapons to 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.