The State Department’s special adviser on Syria told reporters Thursday that Washington has “lots of evidence” Bashar al-Assad’s regime is preparing chemical weapons in the final hours before an expected assault on the nation’s last rebel stronghold, Idlib.

Jim Jeffrey told reporters that he “very sure that we have very, very good grounds to be making these warnings,” according to Reuters.

“Any offensive is to us objectionable as a reckless escalation,” he explained. “There is lots of evidence that chemical weapons are being prepared.”

Observers fear that the Assad regime would use such weapons in the upcoming siege by Russian and Syrian forces on the region, one of the few remaining rebel strongholds. Last week, State Department spokeswoman Heather Nauert warned both countries that the United States would “respond swiftly and vigorously” to the use of chemical weapons.

At a press briefing, she said the U.S. had made it “very clear that the United States government and its partners will respond to any verified chemical weapons use in Idlib or elsewhere in Syria for that matter in a swift and appropriate manner … So we would encourage Russia to make this point very clear to Damascus that that will not be tolerated.”

Jeffrey went on to describe the situation in Idlib as “very dangerous” but said that Turkey, a country allied with the rebel forces, was attempting to reach a deal that would avoid an all-out siege from the government forces.

“I think the last chapter of the Idlib story has not been written,” he said. “The Turks are trying to find a way out. The Turks have shown a great deal of resistance to an attack.”

In April, President Donald Trump ordered precision missile strikes on Syria in response to Assad’s alleged use of chemical weapons with the support of both British and French forces.

“The purpose of our actions tonight is to establish a strong deterrent against the production, spread and use of chemical weapons,” he said at the time. “Establishing this deterrent is a vital interest of the United States.”

The U.S. has largely backed off the idea of enforcing regime change in Syria, although troops still remain in the region to help root out the remaining Islamic State militants. Jeffrey indicated that although Assad “has no future as a ruler,” the U.S. would work with Russia to ensure a political transition.

“Right now (the Syrian government) is a cadaver sitting in rubble with just half the territory of Syria under regime control on a good day,” he said.

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