International calls mounted yesterday to avoid a "massacre" by regime forces in Syria's last rebel-held province of Idlib, two days before a summit between key powers backing the government and opposition.

Troops have been massing on the edges of the northwestern province on the border with Turkey for weeks, raising fears of a humanitarian disaster on a scale not yet seen in Syria's seven-year conflict.

Turkey's President Recep Tayyip Erdogan was the latest to warn Damascus against an all-out offensive against a region the UN says is home to nearly three million people.

"God forbid, a serious massacre could take place if there is a rain of missiles there," said Erdogan yesterday, whose country has supported Syrian rebels.

He spoke two days before he is set to meet the presidents of regime backers Iran and Russia in Tehran to discuss the future of the province.

Global concern has risen in recent days over a threatened regime assault to oust rebels and jihadists from Idlib province and surrounding areas, the last major chunk of Syria still in opposition hands.

On Tuesday, the UN peace envoy for Syria urged Erdogan and Russia's President Vladimir Putin to speak on the phone before Friday's summit. Staffan de Mistura called for efforts "to avoid that the last probably major battle of the Syrian territorial conflict... ends in a bloodbath".

More than half of Idlib is controlled by jihadists from Syria's former al-Qaeda affiliate, Hayat Tahrir al-Sham (HTS), while much of the rest is held by rebels backed by Turkey.

The regime holds a small southeastern sliver.

The United Nations and aid groups have warned a military campaign could spark one of the worst humanitarian catastrophes in a war that has already killed more than 350,000 people and displaced millions.

Some 2.9 million people live in Idlib and surrounding areas, among them one million children.

Many are rebels and civilians who were bussed out of their hometowns in other parts of the country that have come back under regime control.

Late Monday, US President Donald Trump also warned against a full scale assault on Idlib, which he said could trigger a "human tragedy".

On Tuesday, Russian warplanes resumed air strikes on Idlib after a 22-day pause. Air raids across the province killed at least 13 civilians, including six children, the Syrian Observatory for Human Rights said.

Yesterday, regime artillery and rocket fire targeted several areas of the province.

Friday's summit in Tehran between key power brokers Erdogan, Putin and Iran's President Hassan Rouhani is expected to determine the scope and timing of any assault on Idlib.

A UN Security Council meeting is also set to be held the same day to discuss Idlib, US ambassador to the United Nations Nikki Haley has said.

She warned on Tuesday against the use of any chemical weapons in Idlib, after the White House pledged to "respond swiftly and appropriately" to any such attack.

Meanwhile, Russia has been accused rebels, with help of west, of preparing for chemical attacks in Idlib to give a pretext to Us-led air strikes on government forces.