President Donald Trump has called for Russia to end its support for the Syrian regime's violence in the Idlib region, the White House said on Sunday.
Backed by Russian air power, Syrian President Bashar Al Assad made significant gains on Sunday as he intensified his assault on the last rebel-held enclaves in the northwestern province of Idlib.
In a call with Turkish President Recep Tayyip Erdogan, Mr Trump "expressed concern over the violence in Idlib, Syria and ... conveyed the United States' desire to see an end to Russia's support for the Assad regime's atrocities."
Russia, along with Iran, have heavily backed Mr Al Assad in the civil war.
A Turkish delegation headed to Russia on Monday to discuss the military escalation in Syria’s Idlib region, where government forces killed more than a dozen Turkish soldiers in two attacks this month.
Turkey's foreign minister also pressed his Russian counterpart over the attacks by Damascus on the last rebel-held bastion in the country.
Turkey has 12 observation posts in Idlib as part of a 2018 deal reached between Ankara and Moscow to prevent a regime offensive, but Syrian regime forces have pressed ahead regardless.
Four of the Turkish posts are believed to be encircled by Syrian forces, and Ankara has threatened to attack Damascus if they do not retreat by the end of February.
"I stressed that the attacks in Idlib must stop and it was necessary to establish a lasting ceasefire that would not be violated," Turkish Foreign Minister Mevlut Cavusoglu told journalists at the Munich Security Conference, after he met with Russian Foreign Minister Sergey Lavrov.
The Syrian government advance also appeared to put the provincial capital of Aleppo out of the firing range of opposition groups for the first time in years.
Thirty villages and towns around the city in the western Aleppo countryside were captured by Syrian forces on Sunday, consolidating the government’s hold over Aleppo province the state news agency Sana reported.
The armed opposition is now squeezed into a shrinking area of nearby Idlib province.
War monitor the Syrian Observatory for Human Rights on Sunday said regime forces "were in control of all the villages and small towns around Aleppo for the first time since 2012."
The fighting in the Aleppo and nearby Idlib province has started a humanitarian crisis
More than 800,000 civilians of nearly 4 million have been displaced, living in open fields and temporary shelters in harsh winter conditions.
Inside an unofficial camp in Syria's Idlib province
Regime forces have for weeks been making gains in northwestern Syria and chipping away at territory held by jihadists and allied rebels, focusing their latest operations on the West of Aleppo province.
Turkey has sent thousands of troops and equipment into the opposition enclave to try to stall the government advance.
Turkey, which backs the opposition, has called for an end to the Syrian government offensive. It also fears that the thousands of people displaced by the fighting may overwhelm its borders.
On Sunday, the Turkish Defence Ministry said that a car-bomb attack by the Kurdish YPG militia had killed two people and wounded five in the north-eastern Syrian town of Tel Abyad, near the Turkish border.
The Arab town, which Turkish-backed forces pushed the YPG from in a major campaign last October, has experienced a series of car blasts that have killed dozens of civilians.
Turkey views the YPG as a terrorist organisation linked to Kurdish insurgents on its own soil.
