Turkey may launch a military operation in Syria's northwestern Idlib province unless fighting there is quickly halted, President Recep Tayyip Erdogan has said.

Meanwhile, an exodus has shaken the region, the last rebel stronghold in Syria's nearly nine-year war, as hundreds of thousands push towards Turkey to escape a sudden and fast-moving advance by government forces.

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Syrian President Bashar al-Assad's forces, backed by Russian air power, have made rapid advances in Idlib.

"We will not allow the regime's cruelty towards its own people, with attacks and causing bloodshed," Erdogan said on Friday.

"Turkey with complete sincerity wants Syria's stability and security, and to this end, we will not shy away from doing whatever is necessary, including using military force."

The recent campaign has raised tensions between Ankara and Moscow, which back opposing sides in the conflict.

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Turkey has 12 military observation posts around Idlib, set up under a 2017 agreement with Russia and Iran, and several of them have since been surrounded by advancing Syrian government forces.

Erdogan accuses Russia of violating agreements to reduce the fighting in Idlib, a charge Moscow denied on Friday.

Kremlin spokesman Dmitry Peskov said Idlib was a haven for fighters targeting Syrian troops and a Russian airbase in Syria.

Influx of refugees

Turkey, which already hosts 3.6 million refugees from Syria, fears a fresh wave of migrants from Idlib.

A United Nations report on Thursday estimated that 390,000 people have fled northwest Syria between December 1 and January 27, 80 percent of them being women and children.

Erdogan on Friday repeated that Turkey could not handle a fresh influx of migrants and would not allow new threats near its borders, even if it meant resorting to military power as it did in three previous cross-border operations in northern Syria.

"We will do what is necessary when someone is threatening our soil. We will have no choice but to resort to the same path again if the situation in Idlib is not returned to normal quickly," he said.

"We will not allow the regime to put our country under the constant threat of migrants by tormenting, attacking, spilling the blood of ... its people."

Turkey, which has backed rebels fighting to remove Assad, has repeatedly asked him to step down, even while Iran, Russia and Turkey have said they seek a political solution to the conflict.