The top general of the United States says he was involved in a "routine dialogue" with President Donald Trump over military options, should Syria ignore Washington's warnings against the use of chemical weapons in an alleged assault on the enclave from Idlib.
Navy General Joseph Dunford, chairman of the Joint Chiefs of Staff, said the United States had not made a decision to use 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 case chemical weapons are used," he told reporters during a trip to India.
"He expects us to have military options and we have provided updates on the development of these military options," Dunford added later.
Syrian President Bashar al-Assad has concentrated his army and allied forces on the front lines in the northwest, and Russian planes have joined the bombing of rebels there, in a prelude to a widely expected assault, despite the objections of Turkey.
This week, a senior United States envoy said there was "a lot of evidence" that Syrian government forces were preparing chemical weapons in Idlib.
The White House has warned that the United States and its allies would respond "quickly and vigorously" if government forces used chemical weapons in Idlib.
Trump has twice bombed Syria for its alleged use of chemical weapons, in April 2017 and April 2018.
The top military official in France also said last week that his forces were prepared to carry out attacks against Syrian targets if chemical weapons were used in Idlib.
Idlib is the only remaining important fortress of the insurgents and a government offensive could be the last decisive battle of the war.
The presidents of Turkey, Iran and Russia did not agree on Friday a ceasefire to prevent 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.
When asked if there was still the possibility of avoiding the assault on Idlib, Dunford said: "I do not know if there is anything that can stop him."
"It's certainly disappointing, but perhaps not (surprisingly) that the Russians, the Turks and the Iranians have not been able to find a solution yesterday," he said.