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The northwestern province is the last major area still outside regime control, after a string of ferocious assaults this year put most of the country back in government hands.

Regime ally Russia and rebel backer Turkey agreed to establish the buffer zone in September.

It was due to come into force in mid-October but some clauses of the plan have been delayed, as repeated clashes erupted between jihadists, rebels and government forces.

At dawn on Thursday, jihadists from Hayat Tahrir al-Sham, a faction led by former Al-Qaeda fighters, attacked a regime position in the east of the province, said the Britain based Syrian Observatory for Human Rights.

"Four regime fighters were killed in the assault, and a member of HTS also died," said Observatory director Rami Abdel Rahman.

"There is an ongoing exchange of artillery fire between the two parties" in land that is part of the planned demilitarised zone, he added.

The agreement between Russia and Turkey to create the buffer zone was aimed at staving off a regime military assault on the province.

As set out under the deal, rebel groups announced they had withdrawn heavy artillery from the zone, but jihadists including HTS refused to leave the area.

Artillery fire by regime forces killed at least seven civilians in Idlib on Friday last week, the Observatory said.

Syria's Foreign Minister Walid Muallem expressed dissatisfaction this week with the implementation of the agreement, and criticised Turkey for shortcomings.

"Terrorists are still present in this sector with their heavy weapons," he was quoted as saying by the official SANA news agency on Monday, referring to the buffer zone.

"This indicates that Turkey does not want to respect its obligations," he added.

Syria's war has killed more than 360,000 people since it erupted in 2011 with the brutal repression of anti-government protests.