Tens of thousands of people are pushing towards Turkey to escape a fast-moving advance by government forces in northwest Syria's Idlib province, the last rebel stronghold in the country's nearly nine-year war.

Backed by heavy Russian air attacks, President Bashar al-Assad's forces have recaptured dozens of towns in the last 10 days in a major campaign that has stoked tensions between Ankara and Moscow and raised the spectre of a new refugee crisis.

UNICEF said on Saturday that violence this week forced 6,500 children to flee daily, bringing the total number of displaced children in northwest Syria to more than 300,000 since early December.

Russia and Syria say they are fighting rebels who have stepped up attacks on civilians in Aleppo in northern Syria, but rights groups and rescue workers say air attacks and shellings have demolished hospitals, schools and homes.

Turkey, which fears a fresh wave of refugees into its territory, adding to the more than 3.6 million Syrians already there, said on Friday it would not tolerate new threats near its border and would act militarily if needed.

Trucks packed with civilians, along with their furniture and personal effects, were seen moving out of towns across much of Idlib and western Aleppo over the past week.