A Turkish official said Saturday that Turkey destroyed a chemical warfare facility after dozens of its soldiers were killed by Syrian regime fire in the last-rebel enclave of Idlib province.

President Recep Tayyip Erdogan on Wednesday vowed Turkey would not take the "smallest step back" in an escalating stand-off with Damascus and Russia over the northern Syrian region of Idlib.

Regime air strikes and artillery fire in northwestern Syria, including on schools in the city of Idlib, killed 19 civilians on Tuesday, a war monitor said.

A Turkish soldier was killed in Syria's Idlib region in a bomb attack by Russian-backed government forces, the defence ministry said on Saturday

Syrian aid workers issued an urgent call for a ceasefire and international help for nearly a million people fleeing a regime onslaught in the country's northwest on Wednesday.

Government air strikes have hit hospitals and displaced persons camps in northwest Syria and killed about 300 civilians as President Bashar al-Assad's forces press an assault

Turkey said on Saturday it had fulfilled its responsibilities in Syria's Idlib region in line with de-escalation agreements with Russia and Iran, warning it would take military action in the area if diplomatic efforts with Moscow fail.

Turkish President Tayyip Erdogan said on Wednesday his military would strike Syrian forces by air or ground anywhere in Syria

Tensions escalated Tuesday between Syria's regime and rebel-backer Turkey as a Syrian military helicopter was shot down and Ankara warned of a "heavy price" for any attacks on its forces.

Syrian regime artillery killed five Turkish soldiers in Idlib Monday, Turkey's defence ministry said, as the number of people displaced by violence in the region neared 700,000 since December.

Syrian regime forces Sunday were set to retake a key motorway connecting the capital Damascus to second city Aleppo following weeks of battles in the rebel-held Idlib region, a monitor said.

The Syrian regime will claw back the country's last major opposition bastion in stages, experts say, chipping away at Idlib until militants and rebels have run out of options.