WASHINGTON — The State Department said on Tuesday that the Syrian government might be renewing its use of chemical weapons, citing a suspected chlorine attack in northwest Syria, and maintaining that any use of such weapons would lead the United States and its allies to “respond quickly and appropriately.”

The Trump administration said the suspected attack took place on Sunday in the Idlib area, the final stronghold of rebels who have tried to topple the government of President Bashar al-Assad of Syria.

In the fall, Mr. Assad appeared to be on the verge of ordering an invasion of the area, where millions of civilians live or are seeking safe harbor, before a cease-fire agreement was put into effect between rebel groups and the Syrian government. But since April, the Syrian military, with help from Russian forces, has started an air bombing campaign in Idlib to retake the territory in what is expected to be a slow and bloody campaign.

“Unfortunately, we continue to see signs that the Assad regime may be renewing its use of chemical weapons, including an alleged chlorine attack in northwest Syria on the morning of May 19, 2019,” Morgan Ortagus, a State Department spokeswoman, said in a written statement on Tuesday.