A renewed drive by President Bashar al-Assad to recapture rebel-held territory in Syria's northwest sparked a new exodus of tens of thousands of civilians towards Turkey's border on Monday.

Syrian forces backed by Russian air power have stepped up a campaign to take control of Idlib province, the last rebel stronghold where millions took refuge after fleeing other parts of Syria earlier in its nearly nine-year civil war.

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The UK-based Syrian Observatory for Human Rights said government forces reached the eastern, northern and southern outskirts of Idlib's Maaret al-Nouman city.

The Syrian army encircled and was close to capturing Maarat al-Numan, an urban centre 33km (20 miles) south of Idlib city. This would mark a significant advance for Assad's drive to take back all of Syria, and end the conflict that has killed hundreds of thousands of people.

The Syrian Observatory has put the number of displaced since January 15 at 120,000.

The renewed fighting comes despite a January 12 ceasefire between Turkey and Russia, which back opposing sides in the conflict.

'Completely destroyed'

A rescue worker who posted a video from Maarat al-Numan said the city had been devastated by an assault of barrel bombs, missiles and shelling in recent days, which had laid waste to scores of homes and vital infrastructure.

"Marat al-Numan is completely destroyed and its population has been displaced and is living in uncertainty," said the civil defence force worker, who did not identify himself.

Raed Saleh, head of the rescue team White Helmets, tweeted that thousands of people fled their homes towards "the unknown" in order to escape the continuing military operation in the countryside of

Idlib.

Moscow and Damascus say they are fighting armed rebels that have stepped up attacks on civilians in Aleppo, but rights groups and rescue workers say air attacks have demolished hospitals, schools, and other civilian areas.

Millions more?

Fouad Sayed Issa, an aid worker with the Violet Organisation in northern Syria, said Assad's latest campaign has frightened Syrians in the rebel enclave who fear death or arrest if their towns are recaptured.

"Over the past few days we have seen thousands of new internally displaced persons - and we are talking here at the very least about 50,000 over the past four days," Issa said.

A witness said thousands on Monday fled from the Idlib towns of Ariha and Saraqib with trucks and cars seen crawling in gridlocked traffic towards areas, including the town of Azaz, close to the Turkish border.

Turkey, which backs some rebel groups opposed to Assad, already hosts more than 3.5 million Syrian refugees and fears millions more could soon cross the border.