HANGZHOU, China -- The U.S. and Russia are near completing a groundbreaking deal that involves military and intelligence cooperation to fight al Qaeda and ISIS-linked terrorists in Syria. That is in exchange for Russia agreeing to pressure Syrian dictator Bashar al- Assad to stop bombing civilians and allow in humanitarian aid to all starving areas.

The potential deal would ground Assad’s own air force to prevent it from continuing its bombing and chemical attacks on civilians and U.S.-backed rebels. In turn, food and medical aid deliveries will be allowed into besieged cities like Aleppo, where hundreds of thousands of civilians have been trapped due to the joint assaults by Syrian and Russian forces.

Syrian opposition sources tell CBS News that U.S. Envoy to Syria Michael Ratney has circulated a letter notifying their political leadership that the U.S. has reached an understanding with Russia. Having been burned by multiple failed attempts at brokering a successful deal with Russia, American diplomats caution that the diplomacy is in flux.

After many failed attempts at convincing Russia to abandon its support for Assad, this is a last-ditch effort by the Obama administration to stop the killing and the flow of refugees from Syria. An estimated 5 million Syrians refugees have fled the country during the past six years of war while more than 400,000 Syrians have been killed.

Begrudgingly, U.S. officials admit that this potential deal is a compromise that puts Russian President Vladimir Putin back at global player status. Now the question is whether he will live up to his end of the bargain. All previous attempts at a diplomatic solution to the military conflict have failed at the expense of Syrian casualties. Syrian opposition sources say they remain skeptical that this latest deal will stick and worry that it will only solidify Assad’s hold on power.

White House officials do expect both Presidents Obama and Putin to meet sometime in the next 48 hours on the sidelines of the G20 summit now underway in China. Their chief diplomats – Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov – are working behind the scenes to finalize the deal here in Hangzhou.

Another wildcard is Assad’s patron Iran, which has its own military advisers and forces on the ground in addition to Iranian-backed Hezbollah fighters. It is unclear whether Moscow has persuaded Tehran to lessen civilian casualties and assaults on U.S.-backed Syrian rebels.

Yet another complication on this chessboard is the recent escalation of fighting involving Turkish troops battling both ISIS and U.S.-supported Kurdish fighters. On Sunday, Mr. Obama will try to tackle that front when he sits down for a bilateral meeting with Turkey’s President Recep Tayyip Erdogan.