(Mohammad Hannon/AP Photo)

Sen. John McCain, R-Ariz., has crossed the Syrian border to meet with rebel forces today, ABC News has confirmed.

McCain, one of the strongest critics of President Obama's handling of the bloody two-year old conflict between Syrian President Bashar al-Assad and rebel forces, is the highest-ranking U.S. official to visit Syria since the civil war began.

The trip was first reported by The Daily Beast's Josh Rogin.

McCain traveled across the border from Turkey to Syria with Gen. Salem Idris, leader of the rebel forces known as the Supreme Military Command. Idris has become the principal interlocutor between the international community and the highly localized Syrian rebel movement.

ABC News confirmed that in meetings with rebel forces during McCain's several-hour visit to Syria, rebel leaders called for more support from the U.S., including assistance in the form of heavy arms, airstrikes against the Syrian national army, and the enforcement of a no-fly zone.

McCain has also called for the administration to enforce a no-fly zone and provide additional assistance to the Syrian rebels.

The Obama administration announced in April that it would provide $123 million in non-lethal aid to the rebels.

ABC News' Jeff Zeleny contributed to this report.