BEIRUT/ANKARA (Reuters) - Russian and Syrian jets hammered a major rebel stronghold on Tuesday, a war monitor said, days before leaders of Russia, Iran and Turkey meet to discuss an expected Syrian government offensive that could spark a humanitarian disaster.

FILE PHOTO:A general view taken with a drone shows part of the rebel-held Idlib city, Syria June 8, 2017. REUTERS/Ammar Abdullah/File Photo

The warplanes bombarded countryside around Jisr al-Shughour on the western edge of the rebel enclave of Idlib after weeks of lull, killing 13 civilians but no fighters, according to the Syrian Observatory for Human Rights and a rebel source.

For President Bashar al-Assad, the defeat of rebels in the northwestern province would mean breaking the last major stronghold of active military opposition to his rule, though other large areas also remain beyond his control.

(GRAPHIC: Syrian army prepares assault on Idlib - tmsnrt.rs/2NHAqh3)

Washington warned Assad against using chemical weapons, promising a swift response if he did.

Since Russia’s entry into the war on his side in 2015, Assad and his other allies, Iran and a group of regional Shi’ite militias, have forced the rebels from a succession of bastions including Aleppo, Eastern Ghouta and Deraa.

A Syrian government minister said the siege of Idlib would probably be resolved by force. “Until now, military action is more likely than reconciliations,” Reconciliation Minister Ali Haidar told Russia’s Sputnik news agency.

Damascus uses the term “reconciliation” for the negotiated rebel surrenders that have taken place in some areas.

“Idlib is different from other regions because of the large numbers of fighters,” Haidar said. “However we cannot say there is no gateway to reconciliation.”

Half of Idlib’s 3 million people have already fled there from their homes in other parts of Syria, according to the United Nations, and any offensive threatens new displacement and human misery.

It could also spark a wider confrontation with Turkey, a supporter of the rebels, whose army has set up observation posts along the Idlib front lines to deter fighting.

Turkey’s Hurriyet daily reported that Turkish armed forces were reinforcing the Idlib border with M60 tanks, and Reuters television filmed a convoy heading toward the border.

Tuesday’s air strikes came hours after U.S. President Donald Trump warned Assad and his allies not to “recklessly attack” Idlib, saying that hundreds of thousands might die.

Trump has twice ordered U.S.-led air strikes against targets in Syria in response to what Washington called the Assad government’s use of chemical weapons against civilians.

“If President Bashar al-Assad chooses to again use chemical weapons, the United States and its allies will respond swiftly and appropriately,” White House spokeswoman Sarah Sanders said on Tuesday.

The U.N. Security Council will discuss Idlib on Friday, Washington’s ambassador to the United Nations said.

“HUMAN SHIELDS”

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The Kremlin dismissed his comments on Tuesday, describing Idlib, where jihadist insurgent factions dominate, as a “nest of terrorism”. Spokesman Dmitry Peskov added: “We know that Syria’s armed forces are preparing to resolve this problem.”

Iran echoed that theme. “Terrorist groups (in Idlib) have mixed with the people,” said Abbas Araqchi, deputy foreign minister, according to the Islamic Republic News Agency. “They are using people as human shields.”

Idlib’s dominant rebel faction is Tahrir al-Sham, an alliance spearheaded by al Qaeda’s former official affiliate in Syria, the Nusra Front, though other groups are also present.

Last week the U.N. envoy to Syria, Staffan de Mistura, said the Nusra Front and al Qaeda, both of which the international body designates as terrorists, had an estimated 10,000 fighters in Idlib.

On Tuesday de Mistura said talks between Russia and Turkey held the key to resolving the fate of Idlib without a bloodbath. He said he had heard reports that Damascus had set a Sept. 10 deadline for diplomacy to work before attacking.

Turkey fears a major assault on Idlib could send a new wave of refugees toward its border, and wants to maintain a “de-escalation agreement” that it struck with Russia and Iran last year.

It has staged two major incursions into Syria, creating a buffer zone along its border in an area north of Aleppo that adjoins Idlib, where it has set up a local administration alongside Syrian rebel groups.

Foreign Minister Mevlut Cavusoglu said Turkey was discussing joint action with Russia to target terrorist groups in Idlib while avoiding a full-scale offensive. Ankara last week added Tahrir al-Sham to its list of designated terrorist groups.

PRETEXT FOR ATTACK?

Ankara has said the presence of radical groups in Idlib is being used a pretext for a military operation. When he was in Russia in late August, Cavusoglu said “we have to differentiate terrorists from other people”, but added that it was also important to eliminate Russia’s concerns.

Iran’s Foreign Minister Mohammad Javad Zarif, on a visit to Syria, told Iranian state television: “Our efforts are for ... the exit of terrorists from Idlib to be carried out with the least human cost.”

He and Peskov said Idlib would be a major subject of discussion at a Sept. 7 summit of Russia’s Vladimir Putin, Turkey’s Tayyip Erdogan and Iran’s Hassan Rouhani.

Last week a source close to Damascus said the government was preparing a phased assault that would initially target the areas in the south and west of the rebel enclave.

Even a staggered offensive would involve fighting around Turkish observation posts, potentially triggering a new escalation in an already complex war.