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 gradual advance has accelerated in recent days on new momentum from heavy Russian bombardments in the crucial 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 anxiously 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 already dealt major blows to the armed opposition and made crucial military gains around the divided city of Aleppo, the provincial capital that was once Syria's largest city and industrial hub.
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 further south, they could surround rebels in Aleppo and employ the type of "starve or surrender" siege the government has used elsewhere.
Assad's forces also broke the insurgents' siege of two towns near Aleppo, Nubol and Zahra, which had survived on government airdrops of food. People there were celebrating Friday and thanking the troops in videos posted on social media.
The government gains have increased the sense of alarm among anti-government insurgents and their civilian supporters, sending thousands of people, including women and children with whatever they can carry, fleeing through orchards.
In one video posted on social media, a woman can be heard calling out: "Russia is bombing us, Iran is bombing us, Daesh" — another name for the Islamic State — "is bombing us. Where should we go?"
A man who said he was from Homs province, several hundred miles to the south, said he had fled to one town after another in Aleppo province, always chased by bombardments and shelling.
"They killed my mother. They killed my father. They killed my brother," he said. "Where are the Arabs? Where is Islam?"
The United Nations said that 20,000 people were stuck at the border fence between Syria and Turkey, and aid groups said as many as 50,000 were expected. Turkish officials have said they will allow refugees to cross, but it was not clear when they would open the border or how many would be allowed through. A few people requiring urgent medical care are being taken to Turkish hospitals.
Opponents of Assad from the area under attack expressed anguish that the government advances had continued while talks were set to take place in Geneva. Yaser al-Hajj, an activist who has often helped foreign journalists in Aleppo, left the talks before they were halted when he heard of the intensifying attacks not far from his hometown, Marea.
"I cannot stay here," he said at the airport in Geneva before he left. "Why is the United States letting this happen? They are letting Russia do whatever it wants. This will help the terrorists."
Hajj was referring to fighters for the Islamic State, also known as ISIS, which he said would benefit from the losses suffered by rival insurgents. Russia has said it sees no distinction between the Islamic State and other insurgent groups.
Josh Earnest, the White House press secretary, acknowledged in a briefing Friday that Aleppo's fall "would be a setback to the opposition." He said the present military campaign promised to worsen the humanitarian crisis in the area, and he said it was likely to delay a political resolution to the conflict.
Rebel groups north of Aleppo, near the border with Turkey, have been holding off Islamic State militants who want to expand into that area to regain access to the border, which they lost to Kurdish-led fighters supported by the United States.
Kurdish militias also want to connect the two enclaves they control near the border to the east and west. That is where the United States and Turkey sought last year to create a "safe zone" for refugees, free of Islamic State fighters. That plan broke down when the United States and Turkey, NATO allies, disagreed on the details.
Turkey would be particularly troubled if the Kurds, whom it considers its main enemy in the area, further encroached on the border area.
It would be unusual, though, for Turkey to send in troops without consulting with the United States, even though their relationship has been strained by Turkey's frustration that the United States is more focused on battling the Islamic State than Assad.
A Saudi official said Wednesday that the kingdom would consider sending troops to Syria to fight the Islamic State, but it appeared he had spoken without consulting with regional allies, and it was not clear what kind of deployment he meant or where it would be.
In neighboring Idlib province, west of Aleppo, insurgents were on alert for shortages and price increases because of the cut in the Aleppo supply road, according to an anti-government activist in the area.
"Today, I wanted to buy salt," said the activist, Mohammad Moataz. "The guy told me: Starting the middle of this month, no more salt. No cars are coming from Aleppo. The bakers couldn't find salt for their bakeries."
The blockade on the Aleppo road will most likely force cargo to be rerouted through Idlib province, where the territory around the border crossing with Turkey is controlled by the Qaida-linked Nusra Front. That "will probably give the jihadists greater leverage" over other rebel groups, wrote Columb Strack, a senior analyst at IHS Country Risk.
Bombardments in the area may indicate that government forces plan on pressing south to cut the Idlib road as well and surround Aleppo.
"This would provide the government with substantial leverage to eventually negotiate over the future of the city from a position of strength," Strack wrote. "However, the remaining population in rebel-held districts of the town would probably face a humanitarian crisis, as already seen in other besieged towns such as Madaya, but on a much larger scale."
Hwaida Saad contributed reporting from Beirut, Maher Samaan from Paris, and Ceylan Yeginsu from Izmir, Turkey.