Sen. John McCainJohn McCainMcCain says he hasn't met with Trump since inauguration Overnight Defense: General warns State Department cuts would hurt military | Bergdahl lawyers appeal Trump motion | Senators demand action after nude photo scandal Senate lawmakers eye hearing next week for Air Force secretary: report MORE (R) traveled to war-torn Syria on Monday to meet with rebel leaders, a spokesman for the Arizona senator confirmed to The Hill.
McCain's visit makes him the highest-ranking U.S. official to enter the country since its civil war began two years ago.
McCain, a leading critic of the Obama administration's policy towards Syria, has been calling for the U.S. to provide lethal aid to opposition forces seeking the ouster of Syrian President Bashar al-Assad.
The administration has resisted calls from lawmakers to arm the rebels over fears those weapons could fall into the hands of Islamist or anti-U.S. elements. Many Syrian rebel groups have been linked to al Qaeda and other terrorist groups.
Last week, however, the Senate Foreign Relations Committee approved a bill to arm the Syrian opposition, a bipartisan rebuke to the White House.
Supporters of the measure say it is necessary to help end a conflict which has already claimed 80,000 lives, according to some estimates.
The White House, though, has continued to focus on a diplomatic solution to ease Assad from power.
More from The Hill:
• Sebelius on hot seat ahead of ObamaCare rollout
• Obama embraces role as consoler in chief to stay above the fray
• Lawmakers press on with IRS probe over recess
• Tech industry targets GOP senators on immigration
• Dems prepare game plan for Benghazi
• Rep. King 'offended' by 'moral anguish' in Obama drone speech
• Rand PaulRand PaulTrump, GOP fumble chance to govern GOP senators pitch alternatives after House pulls ObamaCare repeal bill Rand Paul takes victory lap on GOP health bill MORE: Obama didn't address due process for drones
Secretary of State John KerryJohn KerryCongress, Trump need a united front to face down Iran One year ago today we declared ISIS atrocities as genocide Trump’s realism toward Iran is stabilizing force for Middle East MORE will meet his Russian counterpart on Monday in Paris to discuss efforts to further peace talks. Russia, one of Syria's staunchest allies, has blocked past attempts in the past to pressure Damascus.
Kerry is also pushing Syrian opposition leaders to negotiate with Assad’s regime during an international conference scheduled for June.
According to reports, rebel leaders asked McCain for heavy weapons from the U.S., as well as a no fly zone to limit attacks from the Syrian air force.
Rebel leaders also told McCain that Assad's troops had used chemical weapons in the war, a move which would cross a "red line" for President Obama.
The administration has said it is continuing to investigate prior allegations of chemical weapons use.