BEIRUT - International calls mounted Wednesday 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 on Wednesday was the latest to warn Damascus against an all-out offensive against a region the United Nations 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, 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.

The Pentagon has seen “zero” evidence to suggest that opposition groups in Syria’s Idlib province could use chemical weapons, US Defense Secretary Jim Mattis said Wednesday, despite Russian claims to the contrary.

Speaking to reporters as he headed for talks in New Delhi on Thursday, Mattis rebuffed suggestions that the US may facilitate a chemical attack, then blame Russia and the Syrian regime and use this as a pretext for air strikes.

The US military has already twice conducted strikes against Syrian President Bashar al-Assad’s regime following two alleged chemical weapons attacks, most recently in mid-April with French and British help. Global concern has risen in recent days over a threatened regime assault to oust rebels and militants 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 militants 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”.

Turkey, Russia and Iran last year designated Idlib a so-called “de-escalation zone”, but that deal did not cover former Al-Qaeda affiliate HTS.

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 war monitor said.

Moscow said four of its jets “inflicted strikes by high-precision weapons” on targets belonging to HTS.

On Wednesday, regime artillery and rocket fire targeted several areas of the province including the HTS-held town of Jisr al-Shughur, according to the Britain-based Observatory, which relies on a network of sources inside Syria.

President Bashar al-Assad’s regime has retaken large swathes of the country from rebels and militants since Russia intervened militarily on its side in 2015.

Moscow has accused armed groups in Idlib of sending weaponised drones to attack the Hmeimim airbase that it uses in the adjacent province of Latakia.

On Wednesday, Putin’s spokesman Dmitry Peskov said Idlib was the “subject of increased concern and worry” and was the focus of a flurry of diplomacy.

Just a day earlier, Peskov had slammed Idlib as a “pocket of terrorism”.

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

Russian presidential advisor Yuri Ushakov described the meeting as “very important”.

“The leaders will in particular focus on the situation in the Idlib de-escalation zone, where terrorist remnants are concentrated”, he said, in particular referring to HTS.

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.

Since 2011, Assad’s regime has repeatedly been accused of using chemical weapons, including this year in its battle to retake the former rebel bastion of Eastern Ghouta near Damascus.

Rescue workers accused regime forces of killing more than 40 people with “poisonous chlorine gas” in the town of Douma in April.

The United States, France and Britain carried out punitive strikes in response.