President Tayyip Erdogan said on Monday Turkey will continue to retaliate after Syrian government shelling killed four of its troops and wounded nine in Syria's northwest Idlib region, where Turkish forces have bulked up in recent days.

Erdogan said initial indications show 30-35 Syrians were "neutralized" in Turkey's response to what the government called intense shelling of its soldiers. He said Turkey has told Russian counterparts "they need to stand aside" in the escalating conflict.

Syrian President Bashar al-Assad's forces, backed by Russian air power, have recently made advances in Idlib, prompting Turkey to warn it may launch a military operation there unless the fighting is halted.

"We have responded in kind to these attacks and will continue to do so, whether it is with our artillery or mortars," Erdogan told reporters in Istanbul.

"We are determined to continue our operations for the security of our country, people and our brothers in Idlib," he said. "Those who question our determination will soon understand they made a mistake."

A Turkish security official told Reuters that the shelling which killed the soldiers occurred in the area of Saraqeb town, 15 km to the east of Idlib city.

"Following the developments in Idlib in recent weeks, serious support was provided over the weekend to the troops, equipment and vehicles in the region," the official said.

The Idlib region is the last major rebel-held stronghold in Syria's nearly nine-year-old war and Erdogan accuses Russia of violating agreements to reduce the fighting in the region, a charge which Moscow denied on Friday.

Turkey, which already hosts 3.6 million refugees from Syria, fears a fresh wave of migrants from Idlib. It has 12 military observation posts around the region, set up under a 2017 agreement with Russia and Iran. Several of them have since been surrounded by advancing Syrian government forces.