Beleaguered rebels in northern Syria faced double defeat on Monday by both the Russian-backed regime and advancing Kurdish militia, as tens of thousands of displaced amassed on the Turkish border.
The worsening refugee crisis has pushed Germany and Turkey to ask NATO for help policing Turkey's shores, after two dozen more migrants drowned there en route to Greece.
In Ankara, German Chancellor Angela Merkel held talks with Turkish leaders, saying the two countries would ask a meeting of NATO defence ministers whether and how NATO could support coastguards.
Merkel said she was "horrified" by the suffering of people stranded on the Syrian-Turkish border after fleeing fighting in northern Syria.
Fierce clashes in the north of Aleppo province -- sparked by a week-long government assault with Russian air support -- have displaced tens of thousands of people.
Turkey's Deputy Prime Minister Numan Kurtulmus said his government fears that violence could drive as many as 600,000 refugees to its border in a "worst case scenario".
The United Nations so far estimates that 31,000 people have fled from areas near and in Aleppo city, a vast majority of them women and children, according to Linda Tom, spokeswoman of the UN's humanitarian aid organisation.
Eight informal camps on the Syrian side of the border are at "full capacity", she said, amid reports that refugees are sleeping rough in fields and on roads.
Government troops and allied forces have seized a string of rebel-held villages, taking them to about 20 kilometres (12 miles) from Turkey.
"It's the first time since 2013 that the Syrian regime has been this close to the Turkish border in Aleppo province," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, a British-based monitor.
- Rebels 'collapsing' -
Kurdish forces have also pushed further east from their bastion in Afrin, seizing six villages in recent days after rebel groups withdrew.
Opposition fighters are now squeezed "between the pincers of the army, which is pushing north, Kurdish forces coming from the west, and IS which dominates the east," Abdel Rahman said.
The regime offensive, one of the largest yet in the north, has cut a major rebel supply route out of Aleppo city.
Regime forces now have their sights set on taking Tal Rifaat, one of three remaining rebel bastions north of the provincial capital.
After capturing the village of Kafeen late Sunday, government forces are now just five kilometres (three miles) south of Tal Rifaat.
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According to analyst Fabrice Balanche, Syria's regime aims to "close off the Turkish border to deprive the rebels of their logistical support".
- 'Total impunity' of regime -
More than 260,000 people have been killed and millions displaced in Syria's bloody war, which diplomatic efforts have so far failed to resolve.
The most recent round of UN-brokered indirect negotiations in Geneva last month was "suspended" until February 25.
Last week, UN Secretary General Ban Ki-moon told The Financial Times newspaper that "aerial bombing continued" when the talks began, making it "extremely difficult" for negotiations to take place.
On Monday, Moscow sharply criticised Ban, saying he had "practically" accused Russia of torpedoing the talks.
"This is clearly not what happened," the foreign ministry said in a statement.
"We have always considered and continue to consider that comments from the chief administrator of an global organisation... should remain impartial and objective."
After talks in Washington between their top diplomats, the United States and Saudi Arabia said Monday they will push for an immediate ceasefire in Syria at international talks later this week.
US Secretary of State John Kerry and Saudi Foreign Minister Adel al-Jubeir met ahead of the broader negotiations in Munich in Thursday.
Both cited UN Security Council Resolution 2254, which calls for a ceasefire and humanitarian access to besieged Syrian towns.
"And we hope that when we meet in Munich in the next few days, we'll be in a position where we can make progress on that goal," Kerry said.
In a scathing report published on Monday, UN investigators accused the Damascus government of "exterminating" prisoners in regime jails and detention centres.
"The mass scale of deaths of detainees suggests that the government of Syria is responsible for acts that amount to extermination as a crime against humanity," commission head Paulo Pinheiro told reporters in Geneva.
"Nearly every surviving detainee has emerged from custody having suffered unimaginable abuses."
Based on 621 interviews -- many of them with former detainees who witnessed deaths while in custody -- the report adds to a huge body of evidence from the commission and others, detailing horrific abuse, torture and killings in Syrian-run jails.