BEIRUT — Syrian government and allied forces pressed their most dramatic advance in months Friday, sending insurgents scrambling and tens of thousands of civilians fleeing toward the border with Turkey.
The advance has accelerated in recent days, with new momentum from heavy Russian bombardments in the northern province of Aleppo. The government's gains have given a morale boost to loyalists and prompted opponents of President Bashar Assad, including Turkey and Saudi Arabia, to calculate their next moves.
The government's gains in Aleppo province, building on earlier ones in Daraa in the south and Latakia in the north, also scuttled U.N.-mediated peace talks this week in Geneva. Neither side saw much to discuss there: The government believed it was achieving its goals on the battlefield, while the opposition accused the Assad administration and Russia of using negotiations as cover for indiscriminate attacks.
Russia's four months of escalating military intervention have strengthened the government, allowing Assad's forces to go on the offensive in several provinces at once for the first time in years. It remains to be seen if the government's advances will hold, but it has dealt major blows to the armed opposition and made crucial military gains around the divided city of Aleppo.
Government forces and pro-government militias, including the Lebanese group Hezbollah, have cut the main supply route for weapons and humanitarian aid north of the city. If the government and its allies advance farther south, they could surround rebels in Aleppo and employ the type of "starve or surrender" siege the government has used elsewhere.
The United Nations' director of humanitarian operations, John Ging, told the Security Council on Friday that the situation around Aleppo, and the closing of an important border crossing with Turkey, could prevent food and medicine from reaching 325,000 people.
Josh Earnest, the White House press secretary, said the administration was deeply concerned about the worsening humanitarian crisis in Aleppo.
"There's the possibility that government forces backed by the Russians would encircle that city and essentially lay siege to that city, and that would obviously exacerbate a terrible humanitarian situation there," Earnest said.