A Russian-backed regime onslaught in northern Syria was reported Wednesday to have killed more than 500 people this month, as Turkey faced new pressure to open its border to people fleeing the violence.
World powers urged Russia to end its air strikes, which a senior US official said were "directly enabling" the Islamic State group, and the UN Security Council met to discuss the conflict.
The meeting behind closed doors came ahead of crucial international talks Thursday in Munich to push for Syrian peace negotiations.
The Syrian Observatory for Human Rights said 506 people had died since the regime launched a major offensive against rebels in Aleppo province on February 1, including 23 children killed in Russian air strikes.
Tens of thousands of Syrians were still stranded Wednesday at the Oncupinar border crossing to Turkey, which remained closed.
Turkish Prime Minister Ahmet Davutoglu on Wednesday said it was building a new camp for the refugees inside Syria, and said it was unfair to ask Turkey to open its borders without pressuring Russia over its bombing.
"I find it hypocritical that some circles are telling Turkey to 'open your borders' while at the same time failing to tell Russia 'enough is enough'," Davutoglu told reporters.
Turkey is already hosting 2.7 million Syrian refugees and has refused to let a new wave into the country, leaving many sleeping in tents or the open.
Medical aid group Doctors Without Borders (MSF) said Wednesday the healthcare system around the war-torn town of Azaz in Aleppo province was "close to collapse" due to the fighting.
Since Saturday, an MSF hospital near Azaz has seen an increase of about 50 percent in its outpatient department. Many are suffering respiratory tract infections.
- 'Dying under bombs' -
Those who have fled the offensive tell of scenes of terror and suffering.
"Children are dying under bombs and from hunger and cold. They are living on the roads. They don't have any place to stay," said Abdul Karim Bahloul.
Human rights groups weighed in, urging Turkey to accept those stranded on its border.
"Forcing people to remain in a war zone, where they risk death and injury, is no solution to the challenge of protecting Syrians fleeing their country," said Human Rights Watch.
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The International Committee of the Red Cross (ICRC) estimated around 50,000 people have been displaced by the violence, mainly in northern areas of Aleppo province.
"The temperatures are extremely low and, without an adequate supply of food, water and shelter, displaced people are trying to survive in very precarious conditions," said from Aleppo Marianne Gasser, the ICRC head in Syria.
Fighting on Wednesday raged around Tamura, north of Aleppo city, with intense Russian air raids on several nearby villages, the Observatory said.
Pro-regime forces have made a series of gains this month in Aleppo province, severing rebel supply lines.
The UN has warned 300,000 people in eastern Aleppo city could be cut off from humanitarian aid if government forces encircle the area, a tactic used by the regime to devastating effect against other rebel bastions.
More than 260,000 people have been killed and half the population displaced since Syria's conflict began in 2011.
- 'Pool of blood' -
UN envoy Staffan de Mistura last week suspended Syria peace talks until February 25 amid opposition protests about the bombing, and the Munich meeting aims to pressure the sides to return to the table.
A key opposition figure insisted the lifting of regime sieges and a halt to air strikes on civilian areas must stop before negotiations can start.
"Before we go on February 25 those measures should be implemented in reality on the land," said Riad Hijab, chief coordinator of the High Negotiations Committee opposition umbrella group.
US President Barack Obama's special envoy to the coalition fighting IS Brett McGurk said Russia's bombing was "directly enabling ISIL," using another term for IS.
On the eve of the Munich talks, Turkish President Recep Tayyip Erdogan hit out at Washington for its support for Syrian Kurds fighting jihadists, accusing it of creating a "pool of blood".
Ankara sees the Syrian Kurdish militia as a branch of the outlawed Kurdistan Workers Party (PKK), but Washington argues the Kurds are key to fighting jihadist groups like IS.
Fighters from the Kurdish People's Protection Units (YPG) and its Arab allies expelled Islamist and rebel fighters from the Minnigh air base and adjacent town, north of Aleppo, the Observatory said.
YPG forces regularly clash with Islamist and jihadist fighters in northern Syria, but its most active front is further east against the Islamic State extremist group.