Tens of thousands of Syrians fled an intensifying Russian assault around Aleppo on Friday, and aid workers said they feared the major city could soon fall under a full government siege.
Iran reported one of its generals had been killed on the front line, giving direct confirmation of the role Tehran is playing along with Moscow in what appears to be one of the most determined offensives in five years of civil war.
The government assault around Aleppo, and advances in the south and northwest, helped to torpedo Geneva peace talks this week. Russia's intervention has tipped the war President Bashar Assad's way, reversing gains rebels made last year.
The last two days saw government troops and their Lebanese and Iranian allies fully encircle the countryside north of Aleppo and cut off the main supply route linking the city - Syria's largest before the war - to Turkey. Ankara said it suspected the aim was to starve the population into submission.
Aleppo would be the biggest strategic prize in years for Assad's government in a conflict that has killed at least 250,000 people and driven 11 million from their homes.
Video footage showed thousands of people massing at the Bab al-Salam crossing on the Turkish border. Men carried luggage on their heads, and the elderly and those unable to walk were brought in wheelchairs. Women sat on the side of the road holding babies and waiting to be allowed into Turkey.
Turkish Prime Minister Ahmet Davutoglu said 15,000 people fleeing Aleppo had arrived at Turkey's border.
Rights group Amnesty International urged the country to let in those fleeing the latest violence.
"It feels like a siege of Aleppo is about to begin," said David Evans, Middle East program director for the US aid agency Mercy Corps, which said the most direct humanitarian route to Aleppo had been severed.
"The situation in Aleppo is a humanitarian catastrophe," said an opposition spokesman still in Geneva after the ill-fated peace talks. "The international community must take urgent, concrete steps to address it."