Damascus - Tens of thousands of Syrians were apparently streaming towards Turkey on Friday as regime troops pressed a major Russian-backed offensive around Aleppo, while Moscow and Ankara traded barbs over the escalating crisis.
Turkey's Prime Minister Ahmet Davutoglu, speaking at a conference in London where donors pledged more than US$10 billion (S$14 billion) in aid for Syrians, said up to 70,000 people were headed towards his country to escape the fighting.
Some 300,000 people are thought to be isolated in Aleppo after the rebels' main supply route was severed by regime forces backed by Russian warplanes in an offensive that scuppered peace talks this week.
The UN Security Council will meet Friday for consultations with envoy Staffan de Mistura over the breakdown of the negotiations, which had been hailed as the biggest diplomatic push to end Syria's five-year war, which have been suspended until February 25.
"The situation in the north countryside of Aleppo is catastrophic," said Maamoun al-Khateeb, an activist and journalist from nearby Marea village.
"Civilians are now besieged from three sides and have just one road to the Turkish territories," he said, explaining that regime forces threatened from the south, Islamic State (IS) jihadists from the east and Kurdish fighters from the west.
As the offensive raged, diplomatic tensions were also rising, with Moscow accusing opposition supporter Ankara of preparing to invade Syria, saying it had spotted troops and military equipment on the border.
Hours earlier Davutoglu had accused supporters of President Bashar al-Assad, which include Russia, of "committing the same war crimes" as the regime.
Western nations have accused Syria's government of torpedoing peace talks this week with its military offensive, and Washington demanded Moscow halt its campaign in support of Assad.
Russian bombings killed at least 21 civilians, including three children, on Thursday, according to Britain-based monitor the Syrian Observatory for Human Rights.
In London for the donors conference, US Secretary of State John Kerry said he had warned Moscow to stop targeting the Syrian opposition, in a "robust" phone call with Russian counterpart Sergei Lavrov.
Meanwhile, Saudi Arabia, which backs opponents of the regime, said it was ready to join any ground operation by the US-led coalition against IS in Syria.
"If there is any willingness in the coalition to go in the ground operation, we will contribute positively in that," Brigadier General Ahmed al-Assiri told AFP.
More than 260,000 people have died in Syria's conflict and more than half the country's population have been forced from their homes, while the chaos has helped to fuel the rise of extremist groups such as IS across the region.
The World Bank on Thursday estimated the war has cost Syria and its neighbours - Turkey, Lebanon, Jordan, Iraq and Egypt - some $35 billion so far. Measured by 2007 prices, that is the equivalent of Syria's entire economic output that year.
Aleppo city, Syria's former economic powerhouse, has been divided between opposition control in the east and regime control in the west since mid-2012.
The rebels' main supply line to Turkey was severed on Wednesday when regime troops broke an opposition siege of two Shiite towns, Nubol and Zahraa, on the route to the border.
Regime forces entered the two towns on Thursday to the cheers of residents, who chanted pro-government slogans and showered the fighters with rice.
But elsewhere in the region the advance prompted tens of thousands to flee for fear of being caught up in the fighting.
Davutoglu said 60,000 to 70,000 people were "moving towards Turkey" and 10,000 were "waiting at the door" on the border because of air strikes and attacks around Aleppo.
The Observatory said nearly 40,000 people in Aleppo province had fled their homes, with many massing at the border.
A high-ranking Syrian government official described the Aleppo advances as important, but said the regime had even more ambitious goals.
"The next objectives are to close the borders with Turkey to prevent the arrival of troops and weapons, then taking Aleppo province, then Idlib province, and finally Idlib city," he told AFP.