US weighing military options if Syria uses chemical weapons in Idlib

The US military is considering options if Syria uses chemical weapons in an impending assault on the enclave of Idlib, despite Washington's warnings.

Air strikes in Syria's southeastern Quneitra province in July forced the remaining rebels to retreat to Idlib in the north. Source: AAP

The United States' top general says he was involved in "routine dialogue" with President Donald Trump about military options, should Syria ignore Washington's warnings against using chemical weapons in an expected assault on the enclave of Idlib.

Marine General Joseph Dunford, chairman of the Joint Chiefs of Staff, said no decision had been made by the United States to employ military force in response to a future chemical attack in Syria.

"But we are in a dialogue, a routine dialogue, with the president to make sure he knows where we are with regard to planning in the event that chemical weapons are used," he told reporters during a trip to India.

"He expects us to have military options and we have provided updates to him on the development of those military options," Dunford later added.

Syrian President Bashar al-Assad has massed his army and allied forces on the front lines in the northwest, and Russian planes have joined his bombardment of rebels there, in a prelude to a widely expected assault, despite objections from Turkey.

Washington says its weighing up military options in Syria. Source: AAP

This week, a top US envoy said there was "lots of evidence" that chemical weapons were being prepared by Syrian government forces in Idlib.

The White House has warned that the US and its allies would respond "swiftly and vigorously" if government forces used chemical weapons in Idlib.

Trump has twice bombed Syria over its alleged use of chemical weapons, in April 2017 and April 2018.

France's top military official also said last week his forces were prepared to carry out strikes on Syrian targets if chemical weapons were used in Idlib.

Idlib is the insurgents' only remaining major stronghold and a government offensive could be the war's last decisive battle.

The presidents of Iran and Russia have backed a military offensive in Syria's rebel-held Idlib. Source: AAP

The presidents of Turkey, Iran and Russia on Friday failed to agree on a ceasefire that would forestall an offensive, perhaps the last major battle in a war that has killed more than half-a-million people and forced 11 million to flee their homes.

Asked whether there was still a chance the assault on Idlib could be averted, Dunford said: "I don't know if there's anything that can stop it."

"It's certainly disappointing but perhaps not (surprising) that the Russians, the Turks and the Iranians weren't able to come up with a solution yesterday," he said.