Turkey called on Russia and Iran on Wednesday to pressure Syrian authorities to halt a military offensive in Syria's rebel-held Idlib province, which Damascus launched despite an international deal to reduce hostilities there.
As pro-government forces pressed the assault, the Russian defence ministry's newspaper said Moscow had asked the Turkish military to tighten control over armed groups in Idlib. It said militants had used the province as the launchpad for a drone attack on two Russian bases in the last week.
Idlib has become a focal point of the Syrian war as government forces and allied militia have thrust towards an insurgent-held air base. Idlib, bordering Turkey, is the largest single chunk of Syria still under the control of rebels fighting President Bashar al-Assad.
The U.N. High Commissioner for Human Rights expressed grave concern for an estimated 2 million people in the Idlib region, where the population has grown as Syrian rebels and civilians fled government advances in other areas. Fresh conflict could trigger more displacement on Turkey's southern border.
Turkish Foreign Minister Mevlut Cavusoglu said Russia and Iran must fulfil their duties under a joint accord reached with Turkey last year in which the three countries announced a "de-escalation zone" in Idlib.
He said advances by the Syrian army and allied forces into Idlib could not have taken place without the support of Moscow and Tehran, which have both backed Assad in the war.
"Iran and Russia need to carry out their responsibilities. If you are guarantors, which you are, stop the regime," Cavusoglu told the state-run Anadolu news agency in an interview broadcast on Turkish television channels.
Cavusoglu's ministry summoned the Iranian and Russian ambassadors on Tuesday to complain about violations of the Idlib de-escalation zone, and he said President Tayyip Erdogan might call Russia's Vladimir Putin on the issue.
Turkey, which has been fiercely opposed to Assad, has recently been working with his allies Russia and Iran for a political resolution to the conflict. But Cavusoglu said the Idlib offensive was endangering those efforts.
"This isn't a simple air strike, the regime is advancing into Idlib. The goal is different here," he said. "If the aim here is to make some unwilling opposition groups go to Sochi, it will backfire," he added, referring to Russia's plans to host a congress on Syria at the end of this month.
In the first attack of its kind, Moscow said on Monday militants had used drones to attack its naval and air bases in the nearby provinces of Tartus and Latakia. The Russian defence ministry's newspaper said on Wednesday the drones had been launched from the south-western part of the Idlib de-escalation zone held by "moderate opposition" armed groups.
Syrian government forces have taken scores of villages in recent weeks near the provincial border between Idlib and Hama, with the help of Iran-backed militias and Russian air power. They have progressed towards the Abu al-Duhur military airport, where rebels completely ousted the army in 2015.
The forces had advanced to within 3 km of the air base on Wednesday, said a military media unit run by Lebanon's Hezbollah, which fights on the Syrian army's side.
The Syrian Observatory for Human Rights said the army and allied militias captured 16 villages and positions on Wednesday. They clashed with Tahrir al-Sham fighters around the airport, the Britain-based war monitoring group said.
The combatants also battled from another direction along a front south of Aleppo city, near the border with Idlib.
Tahrir al-Sham, spearheaded by al-Qaeda's former Syria branch is now the dominant insurgent force in the northwestern province of Idlib.
Rebel groups seized Idlib, which borders Turkey, in 2015 and it has since become the only Syrian province fully under insurgent control.
Fighting and air strikes have forced more than 60,000 people to leave their homes since Nov. 1, according to the U.N. Office for the Coordination of Humanitarian Affairs (OCHA).
"What I am seeing here is the use of the presence of al Nusra and HTS as an excuse to attack civilians and moderate oppositions," Cavusoglu added.
Under last year's deal with Iran and Russia, Turkey says it has deployed troops to observation points in northern Idlib, about 60 km (40 miles) north of the latest Syrian army offensive.
Cavusoglu said Ankara would host a meeting on Syria with like-minded countries after the summit in Russia's Sochi.
With Russian and Iranian backing, government forces have recovered large swathes of territory from rebel factions and Islamic State militants over the past year.