US President Donald Trump has called for Russia to end its support for the Syrian regime's "atrocities" as he expressed US concern over violence in the Idlib region, the White House said.

Turkey's foreign minister also pressed his Russian counterpart over the attacks by the Syrian government on the last rebel-held bastion in the country.

Backed by Russian air power, Syrian President Bashar al-Assad made fresh gains as he intensified his assault on the holdout 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."

Turkey has 12 observation posts in Idlib as part of a 2018 deal reached between Turkey and Russia 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 Turkey has threatened to attack Damascus if they do not retreat by the end of February.

The rebel-held town of Ariha in Idlib province following an air strike earlier this month by pro-regime forces

"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 Russian Foreign Minister Sergei Lavrov.

Rebel supporter Turkey and Syrian ally Russia have worked closely on Syria in recent years despite being on opposing sides of the nine-year conflict.

A Turkish delegation will head to Russia today, after Russian officials visited Turkey last weekend but failed to reach a concrete deal.

The Syrian Observatory for Human Rights said regime forces "were in control of all the villages and small towns around Aleppo for the first time since 2012."

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.

The Russian-backed offensive has triggered the largest wave of displacement in Syria's civil war, with 800,000 people fleeing since it began in December, the United Nations has said. 

Backed by Russia, Iran and the Lebanese Shia movement Hezbollah, Mr Assad's forces now control more than 70% of Syria and the president has repeatedly vowed to retake the entire country.

US President Donald Trump (L) with Turkish President Recep Tayyip Erdogan

In the phone call on Saturday with Mr Erdogan, Mr Trump also "reiterated that continued foreign interference in Libya would only serve to worsen the situation."

Libya has been mired in chaos since a 2011 NATO-backed uprising killed longtime dictator Muammar Gaddafi, with two rival administrations vying for power.

Countries including Russia, France, the United Arab Emirates, and Egypt support strongman Khalifa Haftar, while the UN-recognised Government of National Accord is backed by Turkey and Qatar.