US Senator John McCain has crossed from Turkey into Syria to meet with rebel leaders in the war-torn nation, according to his spokesman.

McCain, the 2008 Republican presidential nominee, slipped across the border on Monday in an unannounced visit lasting several hours before he returned to Turkey, his spokesman Brian Rogers confirmed.

Rogers declined to give any details about the visit.

The visit came as US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov pledged to do their utmost to bring Syria's warring parties together, and new allegations surfaced about chemical weapons use in the civil war.

General Salem Idris, who leads the Supreme Military Council of the Free Syrian Army, told news website Daily Beast in an interview that McCain's visit came at a critical time for the rebels, who have stepped up their calls for US support, including heavy weapons, creation of a no-fly zone and air strikes.

"The visit of Senator McCain to Syria is very important and very useful especially at this time," the publication quoted Idris as saying. "We need American help to have change on the ground; we are now in a very critical situation."

McCain is the highest-ranking US official to visit Syria since Robert Ford, the US ambassador to Syria, crossed the border into northern Syria to meet with Syrian opposition leaders earlier this month.

It was not immediately clear if McCain, a fierce critic of the Obama administration's handling of the Syrian crisis, told government leaders about his plans to visit the country.

The White House had no immediate comment.

A senior Sate Department official, in Paris with Kerry, confirmed that McCain did "cross into Syrian territory" but referred all questions to McCain's office.

Source: Agencies