While the United States, Saudi Arabia and other nations support the rebels, Iran has sent military personnel to help the government, and Russia is bombing opposition forces. And all oppose the Islamic State, which has seized territory spanning the Syrian-Iraqi border.

If the government were to gain control of Aleppo, Syria’s largest commercial center before the war, the loss would represent a major blow for the rebels after being pushed out of other major towns and cities by both the government and the Islamic State. The government’s recent advance makes it clear that Russia’s intervention has not simply stabilized the government’s position, but enabled it to advance, diminishing pressure on Mr. Assad to compromise in negotiations.

The new government advances between the city of Aleppo and the Turkish border were facilitated by heavy Russian and Syrian airstrikes that increased while representatives of the warring parties were in Geneva for peace talks, opposition activists said. Aleppo has been divided between government- and rebel-controlled zones since 2012.

While government forces coming from the east and west had yet to meet and successfully block rebel supply lines on Thursday, fighting and airstrikes in the area had cut off the main route used to deliver humanitarian aid, according to Mercy Corps, an aid group.

“We are cut off from Aleppo city,” David Evans, the group’s regional program director for the Middle East, said in a statement. “It feels like a siege of Aleppo is about to begin.”