A Turkish delegation was in Russia on Monday for talks on Syria, following reports that Russian-backed attacks there were forcing tens of thousands more Syrians to flee towards Turkey.
Turkey already hosts about 3.7 million Syrians - the world's biggest refugee population. President Tayyip Erdogan said on Sunday it could not handle a new influx and was urging Russia to stop the strikes in Syria's northwestern Idlib province.
The Turkey-based Humanitarian Relief Foundation (IHH) said on Monday 120,000 Syrians were fleeing towards the Turkish border - higher than Erdogan's estimate of 80,000.
UN Secretary General Antonio Guterres called for an immediate end to the hostilities, which he said had displaced 30,000 people in the last week alone, his spokesman said.
"The Secretary-General reminds all parties of their obligations to protect civilians and ensure freedom of movement," UN spokesman Stephane Dujarric said in a statement.
Syrian President Bashar al-Assad has vowed to recapture the Idlib region, the last significant area of Syria still under rebel control after eight and a half years of civil war.
Russia and Iran have supported Assad's forces during the Syrian conflict while Turkey has backed Syrian rebels fighting Assad.
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Russian and Syrian army jets have been targeting civilian convoys trying to flee the Idlib city of Maarat al-Numan, leaving hundreds of families still trapped there, activists and aid groups have said.
"It's a tragic situation for civilians remaining in the city since Russian jets are hitting any convoy that leaves the city, while those who were able to reach areas closer to the border have nowhere to shelter," said Mohamad Rasheed, an activist in the area.
Libya also in focus
The Syrian army said on Monday it had fought fierce battles in southeastern Idlib and seized several villages after killing large numbers of what it called terrorists.
"People are lying on the ground sleeping in the open with no blankets and under rain and cold," said Feras Saad, who fled the city with his family and found shelter in the border village of Harabosh.
Moscow and Damascus both deny allegations of indiscriminate bombing of civilian areas and say they are fighting al Qaeda-inspired Islamist militants.
A Turkish diplomatic source said the delegation in Russia will also discuss Turkey's potential troop deployment and military support to Libya, after Ankara and Tripoli signed a military cooperation accord last month.
Russia has said it is concerned about any such deployment.
Erdogan said on Sunday Turkey could increase military support to Fayez al-Serraj's UN-backed Government of National Accord (GNA), which has been fighting off an offensive by east Libya forces led by Khalifa Haftar.
Russia, Egypt and the United Arab Emirates have backed Haftar's forces.
Later on Monday, the Turkish Foreign Ministry said Foreign Minister Mevlut Cavusoglu held a phone call with his Russian counterpart Sergei Lavrov to discuss developments in Syria and Libya. It did not give further details.