A U.S. advisor warned Thursday that there is “lots of evidence” that Syria is preparing chemical weapons as it gears up for an assault on Idlib, the country’s last major rebel-held enclave, Reuters reports.
“I am very sure that we have very, very good grounds to be making these warnings,” said Jim Jeffrey, who was named the State Department’s special adviser on Syria last month. “There is lots of evidence that chemical weapons are being prepared,” Jeffrey told reporters, according to Reuters.
The White House said that the U.S. would respond “swiftly and vigorously” if Syrian forces used chemical weapons in Idlib.
More from TIME
Syrian rebels and civilian refugees fled to Idlib as forces loyal to the government of President Bashar al-Assad retook other “de-escalation” zones in Aleppo and eastern Ghouta. Now, the northwestern territory is the only remaining holdout, and Syrian forces have been observed preparing for what could be the final assault of the country’s seven-year civil war. Approximately 3 million civilians, including 1 million children, are trapped in the rebel-held province, and half of those have been previously displaced, according to the U.N.
Read more: Idlib Could Be the Last Major Battlefield of the Syrian Civil War. But Assad Won’t Take It Easily
Syria began shelling areas on Idlib’s outskirts on Tuesday and resumed joint airstrikes with Russian forces that killed 13 people, including children, according to monitor group the Syrian Observatory for Human Rights. The group reported more airstrikes Friday morning, according to Reuters.
On Thursday, eight European countries urged Syria, Russia, and Iran to respect a ceasefire in Idlib, warning that military confrontation could have “potentially catastrophic humanitarian consequences for civilians.”
On Friday, the presidents of Russia, Iran and Turkey, which supports Syrian opposition forces, will meet in Tehran to negotiate Idlib’s future. Also on Friday, the U.S. will chair a U.N. Security Council meeting about the possible attack on Friday; American U.N. Ambassador Nikki Haley said any military campaign in Idlib would be “a reckless escalation,” the Associated Press reports.