Assad regime forces killed 22 opposition forces overnight near Idlib province, in the deadliest such attack in an area where a recent truce is to be enforced, a monitor said Friday.

Fighting erupted when regime troops seized a position in a rural area in the north of neighboring Hama province that had been held by the Jaish al-Izza group, the Syrian Observatory for Human Rights said.

Idlib and some surrounding areas are the last major opposition bastion in Syria, where the Russian-backed regime has in recent months retaken much of the territory it had lost since the civil war erupted in 2011.

It had threatened an assault on opposition territory, home to around 3 million people, but a deal for a demilitarized buffer zone around it was reached in September between Moscow and Ankara.

Several deadly skirmishes have occurred since the deal but 22 is the highest number of known fatalities in a single incident inside the planned buffer zone, the Observatory said.

"This is the highest death toll in the demilitarized zone since it was announced," Rami Abdel Rahman, head of the Britain-based monitoring group, told AFP.

Last week, at least 10 civilians were killed and dozens more injured in attacks by the regime forces and pro-regime terrorist groups in demilitarized zone.

The regime and Iranian-backed terrorist groups fired artillery into the villages of Jarjanaz and Al-Tamanah, both of which are situated inside the demilitarized zone.

Meanwhile, Russia stated that work to establish a demilitarized zone in Idlib continues and that it is too soon to say that the process has been completed.

Russian Foreign Ministry Spokesperson Mariya Zaharova said on Friday that although there has been practical success implementing the deal, the process still continues. She emphasized that extremist groups, such as al-Nusra Front, a former Al-Qaeda affiliate that is recognized as a terrorist group by Turkey, still engage in provocation activities to prevent forming a demilitarized area.

The buffer zone, agreed between Ankara and regime ally Moscow, aims to separate regime fighters from the opposition in the Idlib region.

The accord, reached on September 17, aims to stave off a massive regime assault on the region by creating a 15 to 20-kilometer (nine to 12-mile) buffer zone ringing the area.

Following a meeting in Sochi last month between President Recep Tayyip Erdoğan and his Russian counterpart Vladimir Putin, the two countries agreed to establish a demilitarized zone in Idlib, Syria's last opposition stronghold.

Ankara and Moscow also signed a memorandum of understanding calling for the "stabilization" of Idlib's de-escalation zone, in which acts of aggression are expressly prohibited.

Under the deal, opposition groups in Idlib will remain in areas in which they are already present, while Russia and Turkey will conduct joint patrols in the area with a view to preventing renewed fighting.

The Sochi deal is not the first attempt by Turkey and Russia to stabilize Syria. The Astana peace process was launched by Russia, Iran and Turkey in January 2017 to bring all warring parties in the Syrian conflict to the table to find a political solution foreseen in U.N. sponsored peace talks in Geneva. The Astana process resulted in an agreement by the three guarantor countries to implement four "de-escalation" zones. The partial cease-fire, however, was short-lived. Regime forces backed by Russia and Iran re-conquered three of the zones through heavy bombardments. Idlib remained as the last stronghold of the opposition with dozens of opposition groups squeezed into the province until the Sochi deal, which gave hope to the region.