An estimated 35,000 civilians and Syrian opposition forces remain virtually surrounded inside Aleppo, previously Syria's largest city and once a major rebel stronghold.

Turkey's Oncupinar border crossing remained closed on Saturday, leaving thousands of Syrians waiting in cold, rainy weather.

Luxembourg Foreign Minister Jean Asselborn called on US Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov to hold urgent talks focused on Aleppo, where a Syrian government offensive backed by Russian airstrikes has prompted more civilians to flee toward Turkey.

"It is highly probable that a large wave of people will now head our way," Asselborn said, referring to a major migratory route via Turkey, Greece and the Balkans amid Europe's wrangle over how to control the EU's external borders.

A Saturday meeting of EU foreign ministers in Amsterdam, billed as informal talks, involved Turkey's Foreign Minister Mevlut Cavusoglu.

Warning from Syria

In Damascus on Saturday, Syrian Foreign Minister Walid Muallem warned Saudi Arabia and Turkey, which back diverse Syrian rebel forces, not to send troops - in the wake of UN-led peace talks that were put on pause in Geneva early this week.

"I assure you [that] any aggressor will return to their country in a wooden coffin, whether they be Saudis or Turks," he warned.

Further international talks on Syria involving key countries are due in Munich next Thursday.

Turkey reminded of 'obligations'

In Amsterdam, EU Regional Policy Commissioner Johannes Hahn publicly reminded Turkey of its obligations under the Geneva Convention to keep its frontier open along its southern border with Syria and "take in refugees."

EU foreign policy chief Federica Mogherini said EU support already provided to Turkey was "aimed exactly at guaranteeing that Turkey" had the resources to host new asylum seekers.

Earlier this week, the EU finalized a 3-billion-euro ($3.4-billion) offer to Ankara to help Turkey house and care for the refugees on its soil in a bid to reduce migratory flows toward Europe.

Visiting Amsterdam, Cavusoglu said Turkey had "received already 5,000" refugees amid the latest exodus and asserted that Turkey would keep "this door open" for fleeing Syrians.

Turkish authorities had earlier said they were first working to free up space within existing refugee camps and finalizing a registration system based on international standards.

The Turkish Red Crescent said it was distributing food and water to those waiting on the border with Turkey.

Tens of thousands

Since Syrian troops stepped up the offensive Monday, some 40,000 civilians have fled Aleppo, according to the Britain-based Syrian Observatory for Human Rights.

A Red Cross spokesman told the French news agency AFP on Saturday that aid delivery access to Aleppo was "difficult."

The governor of Turkey's Kilis border province, Suleyman Tapsiz, said 35,000 refugees were being accommodated and given food at camps on the Syrian side.

Another 75,000 Syrians could be expected if Russian airstrikes and Syrian regime military advances continued in Aleppo province, he said.

Turkey already hosts more than two million people who have fled since Syria's war began in 2011. In London on Thursday, world leaders pledged large sums for refugee assistance in countries neighboring Syria.

Fighting disrupts aid routes

Another 10,000 were thought to have been displaced from the Kurdish town of Afrin in the north of Aleppo province, said the UN's office for the Coordination of Humanitarian Affairs (OCHA) on Saturday.

OCHA spokeswoman Linda Tom said the fighting had disrupted major aid supply routes from the Turkish border.

Russia, which has a Mediterranean naval base in Syria's Latakia province, began its aerial campaign in September - purportedly as part of US-led airstrikes directed at "Islamic State" (IS) jihadists holding swathes of northern Syria and western Iraq.

ipj/sms (AFP, dpa, Reuters, AP)