NEW DELHI — America's top general on Saturday said he was involved in "routine dialogue" with the White House about military options should Syria ignore U.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 a small group of reporters during a trip to India.

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

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.

Witnesses and rescuers said at least a dozen air strikes hit a string of villages and towns in the region on Saturday.

This week, a top U.S. 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 United States and its allies would respond "swiftly and vigorously" if government forces used chemical weapons in Idlib. President Donald Trump has twice bombed Syria over its alleged use of chemical weapons, in April 2017 and April 2018.

Dunford did not say, one way or the other, what he expected Trump to do should Syria use chemical weapons again.

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.

Dunford declined to comment on U.S. intelligence about the possible Syrian preparations of chemical agents.

"I wouldn't comment on intelligence at all, in terms of what we have, what we don't have," he said.

Idlib is the insurgents' only remaining major stronghold and a government offensive could be the last decisive battle in a war that has killed more than half a million people and forced 11 million to flee their homes.

At a summit in Tehran on Friday, the presidents of Turkey, Iran and Russia failed to agree on a ceasefire that would forestall an offensive.

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.

Iranian President Hassan Rouhani at a press conference with Turkish President Recep Tayyip Erodgan and Russian President Vladimir Putin in Tehran after their trilateral meeting on Friday. - / AFP - Getty Images

Tehran and Moscow have helped Assad turn the course of the war against an array of opponents ranging from Western-backed rebels to the Islamist militants, while Turkey is a leading opposition supporter and has troops in the country.

Turkey says it fears a massacre and it can not accommodate any more refugees flooding over its border.

But Russia's Vladimir Putin said on Friday a ceasefire would be pointless as it would not involve Islamist militant groups it deems terrorists.

Dunford has warned about the potential for a humanitarian catastrophe in Idlib and instead has recommended more narrowly tailored operations against militants there. "There's a more effective way to do counterterrorism operations than major conventional operations in Idlib," he said.