Russian and Syrian government forces have intensified an assault on rebel-held areas around the Syrian city of Aleppo that has prompted tens of thousands to seek refuge across the border in Turkey.
The assault around Aleppo, which aid workers have said could soon fall to government forces, helped to torpedo Syrian peace talks in Geneva this week.
Russia's intervention has tipped the balance of the war in favour of President Bashar al-Assad, reversing gains the rebels made last year.
Any hopes of a ceasefire were dampened by Assad's foreign minister, who on Saturday said it would be all but impossible to stop the fighting while rebels were able to pass freely across the borders with Turkey and Jordan.
Taking full control of Aleppo, Syria's largest city before the civil war erupted five years ago, would be a huge strategic prize for Assad's government in a conflict that has killed at least 250,000 people and driven 11 million from their homes.
Advances by the Syrian army and allied militias, including Iranian fighters, are threatening to cut off rebel-held zones of the city, still home to around 350,000 people, while more than a million live in government-controlled areas.
Mevlut Cavusoglu, foreign minister of Turkey, which has already taken in 2.5 million Syrians, said up to 55,000 were now fleeing to the frontier.
Cavusoglu said the border was open, but at the Oncupinar crossing near the Turkish city of Kilis, which has been largely shut for nearly a year, refugees were being shepherded into camps on the Syrian side.
The local governor, Suleyman Tapsiz, said around 35,000 Syrians had reached Oncupinar in the space of 48 hours.
"Our doors are not closed, but at the moment there is no need to host such people inside our borders," he said.
Russia denies targeting civilians and says its actions are aimed at shoring up Syria's legitimate government and combating terrorism. The West and Turkey, which want Assad to step down, accuse Moscow of using indiscriminate force.
But Syria has been emboldened by Russia's intervention.
Foreign Minister Walid al-Moualem told a news conference on Saturday that Damascus would resist anyone who launched a ground incursion into its territory.
"Those (who do so) will return to their countries in coffins," he said, adding that no ceasefire would be possible unless borders were sealed.
The comments appeared to be aimed at Saudi Arabia and Bahrain, which said this week they were ready to participate in any ground operations in Syria that the US-led coalition fighting Islamic State militants in Syria and Iraq decided to mount.