Ankara demanded on Wednesday that Damascus pull back from Turkish-manned posts in Syria's last rebel enclave, after deadly clashes this week triggered plans for emergency UN talks.

The escalation between Turkish and Syrian troops, which killed more than 20 people on Monday in northwestern Idlib province, is testing uneasy relations between Turkey and Russia - the key foreign powerbrokers in the conflict.

The UN Security Council will hold an emergency session on Thursday following requests from the United States, Britain and France, diplomats said on Wednesday.

"If the regime does not pull back, Turkey will be obliged to take matters into its own hands," Turkish President Recep Tayyip Erdogan told a meeting of his ruling party MPs in Ankara, giving Syria until the end of the month to comply. But Syrian regime forces continued to pound Idlib on Wednesday, an offensive which has killed 300 civilians since December and displaced 520,000 people.

The UN envoy for Syria, Geir Pedersen, is expected in New York to report on the situation in Idlib, where the two armies clashed on Monday, the diplomats said.

Syrian troops seized more than 20 towns and villages from rebels and jihadists over Tuesday and Wednesday, according to the Britain-based Syrian Observatory for Human Rights and state news agency SANA. The Observatory said Russian air strikes killed three civilians - all members of the same family - on the outskirts of Idlib city on Wednesday. Regime rocket fire killed another civilian in the town of Anjara in the west of Aleppo province.

Erdogan said two of Turkey's 12 so-called observation posts in Idlib, set up under a 2018 agreement with Russia, were now "behind the regime's lines".

The post at Morek was surrounded by forces loyal to Syrian President Bashar al-Assad in December and another at Surman now lies within the regime's area of control.

Monday's clashes - the bloodiest clashes since Ankara sent troops to Syria in 2016 - prompted Erdogan to call on Moscow, the key backer of Assad's regime, to "better understand our sensitivities in Syria". He spoke with his Russian counterpart Vladimir Putin on Tuesday and told him Ankara would respond "firmly" to any new Syrian attack. Syria's military command said on Tuesday the presence of Turkish forces was "illegal and a flagrant act of aggression", vowing to respond to any Turkish attack on its forces, SANA reported.