Turkey on Thursday threatened to use force against "radicals" in Syria's Idlib province after Russia accused Ankara of failing to "neutralise" jihadist groups under a 2018 deal. 

"Force will be used in Idlib against those who do not abide by the ceasefire, including the radicals," Defence Minister Hulusi Akar was quoted as saying by the official Anadolu news agency. 

"Any form of measure will be taken," he said. 

Idlib -- the last opposition bastion in Syria -- is held by an array of rebels dominated by the Hayat Tahrir al-Sham (HTS) jihadist group, which is led by members of the country's former Al-Qaeda franchise.

President Bashar al-Assad's forces have pressed ahead with an offensive in the region since December, killing more than 380 civilians, according to the monitoring group the Syrian Observatory for Human Rights. 

The killing of 14 Turks in Idlib in government shelling has fuelled tensions between Ankara and Damascus, while raising stakes with Russia -- a key ally of Assad. 

President Recep Tayyip Erdogan on Wednesday accused Russia of committing "massacres" in Idlib and threatened to strike government forces anywhere in Syria if the slightest harm is done to Turkish troops. 

In return, Moscow accused Ankara of failing to honour the 2018 deal, with Kremlin spokesman Dmitry Peskov saying the Turkish side "had taken upon itself an obligation to neutralise terrorist groups" in Idlib.

Under the bilateral agreement, radical groups were required to withdraw from a demilitarised zone in the Idlib region.

Turkish Chief of Staff General Yasar Guler Thursday spoke by phone with his Russian counterpart, General Valery Gerasimov about the situation in Idlib, the Turkish army announced on Twitter.

'No military victory'

As Turkey and Russia traded accusations over the escalation in Idlib, US special envoy for Syria James Jeffrey voiced Washington's support for Ankara's "legitimate" interests in Syria and particularly in Idlib. 

"Our job is to convince (Russia, Iran and the Assad government) that they are not going to have a military victory," Jeffrey said, in comments shared on the embassy's official Twitter account -- a day after he met with Turkish officials in Ankara.  

"The United States totally agrees with Turkey on the legal presence and justification for Turkey defending its existential interests against refugee flow and dealing with terror and finding a solution to the terrible Syrian conflict with the war criminal regime of President Assad," he said.  

Turkey, which already hosts more than 3.7 million Syrians, fears a further influx of refugees fleeing violence in Idlib. 

The two countries have been on a collision course over US support for Kurdish militants fighting against the Islamic State in Syria, deemed as "terrorists" by Ankara. 

Under the bilateral deal with Russia, Turkey has also set up 12 observation posts in Idlib -- three of which were encircled by Assad's forces, according to Turkish officials. 

Erdogan has now given Damascus until the end of the month to push back its forces outside the military locations. 

Turkey has sent reinforcements including troops and artillery to beef up its observation posts in recent weeks following the series of exchanges with the Syrian army.

Akar said: "We are sending additional units to establish a ceasefire and make it long-lasting. We will control the field."