BEIRUT (AP) - Syrian government forces and allied militias, backed by what an aid official said were "hugely increased" Russian airstrikes, pushed ahead with a major offensive in northern Syria on Friday, moving a step closer to encircling contested Aleppo, Syria's largest city.
The offensive near the Turkish border, which began earlier this week, has sent tens of thousands of residents fleeing.
Turkish Prime Minister Ahmet Davutoglu said Friday about 15,000 Syrians reached Turkey's borders and that tens of thousands more could be on the way. Davutoglu promised Turkey would not leave the displaced "without food or shelter" but did not say whether Turkey would admit them.
A Turkish charity said the number of Syrians at one border crossing alone had risen to 50,000 since Thursday.
In recent days, pro-government forces have captured several towns and villages north of Aleppo, driving a deep wedge into rebel-held areas and cutting off a rebel supply road to Turkey.
Aleppo, once Syria's thriving commercial center, has been carved up between government- and rebel-controlled districts since the summer of 2012.
A government siege of Aleppo and its rebel strongholds could isolate tens of thousands of civilians and potentially cut off aid deliveries from Turkey. It would also deliver a devastating blow to the morale of opposition groups fighting for the past five years to topple President Bashar Assad.
In other fighting Friday, the government also retook a town in southern Syria, close to the provincial capital of Daraa.
The Russian bombing campaign, meanwhile, drew new criticism from the West, which has sharply criticized Moscow for its military support of Assad. Airstrikes that mainly target opposition forces are "undermining efforts to find a political solution to the conflict," NATO Secretary-General Jens Stoltenberg said Friday.
Prospects for meaningful peace talks are increasingly dim.
Earlier this week, a U.N.-led attempt to launch indirect talks between a government delegation and opposition representatives in Geneva was adjourned after several days of acrimonious bickering. U.N. envoy Staffan de Mistura announced Wednesday there would be a "temporary pause," but the process will resume Feb. 25.
However, the opposition's chief negotiator, Mohammed Alloush, told the Associated Press late Thursday his delegation is unlikely to return to Geneva because of the "merciless" bombing campaign by Russia and the Syrian air force.
Russia began launching air strikes in Syria in late September, ostensibly to target militants of the extremist Islamic State group, which controls large areas of northeastern Syria. However, critics have said Russian warplanes have struck a wide range of opposition targets.
The Russian defense ministry said it hit 875 targets in bombing raids across Syria this week, including in the area of the current offensive.
The international aid group Mercy Corps, which has been delivering food and other necessities to civilians in northern Syria, had to stop distributions in opposition-held areas of Aleppo earlier this week because the sole access road became too dangerous, said Rae McGrath, head of operations in Turkey and northern Syria.
He said Russian airstrikes north of Aleppo have "hugely increased" in the past two weeks. McGrath said he believes the air strikes "have been targeting areas where you could say this would force people to move."
The Turkish Islamic charity IHH said about 50,000 people arrived since Thursday at the Bab al-Salam border crossing between Syria and Turkey.
Serkan Nergis, a spokesman, said the group is setting up tents on the Syrian side of the border to provide temporary shelter. The charity runs about 10 camps for displaced Syrians along the frontier.