Turkey said it has fulfilled its responsibilities in Syria's Idlib region and warned it would take "necessary steps" if diplomatic efforts with Russia fail, amid a continuing Syrian government offensive on the last rebel-held region in the country.

Ankara, which backs several Syrian rebel groups, and Moscow, which supports the Syrian government, agreed in September 2018 to set up a de-escalation zone in opposition-controlled northwestern Syria.

Under the 2018 deal, Turkey has 12 observation posts in Idlib, with some of them now being in Syrian government-controlled territory following gains by Damascus.

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Turkey's Vice President Fuat Oktay on Saturday insisted Ankara had enforced its side of the agreement.

"Observation posts were set up and the regime had to stay outside of this area. Russia and Iran were to ensure the regime stayed outside, Turkey had responsibilities too, Turkey fulfilled these," Oktay told the NTV broadcaster.

"Undertaking an extremely risky and difficult duty, Turkey took real initiative to stop the bloodshed of civilians, to prevent a new migration wave and to ensure it did not become a terror nest."

Later on Saturday, the Turkish presidency said in a statement that President Recep Tayyip Erdogan and his US counterpart, Donald Trump, discussed ways to end the crisis in Idlib and condemned the attacks by the Syrian government in the region.

"Stressing that the regime's most recent attacks are unacceptable, the president and Trump exchanged views on ways to end the crisis in Idlib without further delay," the presidency said in a statement after the two leaders spoke on the phone.

Escalating tensions

Launched in April last year, the Syrian government offensive has disrupted fragile cooperation between Turkey and Russia. After several failed ceasefire attempts last summer, the Syrian government intensified its assault on the region in December, killing hundreds of civilians and forcing hundreds of thousands to flee.

The situation escalated further this month when 13 Turkish military personnel were killed. Ankara responded by hitting scores of Syrian government targets.

On Friday, a Syrian military helicopter was shot down in the western countryside of Aleppo province, in an attack claimed by rebel groups. The incident came days after rebels said they shot down another government helicopter near the town of Nairab.

According to the United Nations, about one million Syrian refugees are living near the border with Turkey, with camps for internally displaced people (IDPs) already at capacity.

The Russian defence ministry said earlier this week Turkey did not separate "fighters from the moderate opposition from terrorists," referring to an agreed demilitarised zone within Idlib.

President Erdogan said later on Saturday that the situation would not be resolved until Syrian government forces withdrew beyond the borders that Turkey and Russia outlined in the 2018 agreement.

"The solution in Idlib is the [Syrian] regime withdrawing to the borders in the agreements. Otherwise, we will handle this before the end of February," Erdogan said, appearing to bring forward a previously stated deadline of the end of February.

"We would like to do this with the support of our friends. If we have to do it the hard way, we are also up for that," he said.

"Until we clear Syria of terrorist organisations and the cruelty of the [Syrian] regime, we will not rest easy."

'Necessary steps will be taken'

Foreign Minister Mevlut Cavusoglu said a Turkish delegation would visit Russia on Monday to discuss the situation in northwestern Syria.

Cavusoglu said Turkey wants to resolve matters with Russia over Idlib through diplomacy, but will take other steps if necessary.

"If it won't work through diplomatic channels, we will take the necessary steps," Cavusoglu told reporters at the Munich Security Conference, adding that he would meet Russian Foreign Minister Sergey Lavrov later on Saturday.

Ankara says it wants to stop the Syrian government's "aggression" in a bid to stop the deaths of civilians and to prevent a wave of refugees fleeing to Turkey.

Turkey is already home to more than 3.6 million Syrian refugees. "Turkey cannot withstand another migration wave," Oktay said.

Turkish President Recep Tayyip Erdogan warned the Syrian government to withdraw from Turkish-manned posts by the end of February, otherwise Ankara will "take matters into its own hands".

In recent days, Turkey has sent multiple military reinforcements to Idlib, and the Hurriyet daily newspaper on Saturday reported that a 60-vehicle convoy carrying commandos and armoured carriers was sent to beef up the Turkish posts.