GENEVA — A week after Syria’s government agreed to a cease-fire with rebel groups affecting the northwestern province of Idlib, air force jets hit multiple targets on Friday, humanitarian workers said, fueling international fears for millions of civilians crammed into the area.

By early afternoon, aid workers on the ground had counted airstrikes on 13 locations and shelling on half a dozen more in Idlib, as well as strikes on two neighboring provinces. There were few accounts of civilian casualties, but heavy damage was reported at two schools.

“We are really at a desperate point not knowing where it’s going again,” Panos Moumtzis, the United Nations official coordinating humanitarian aid for Syria, said in an interview. “We keep on raising the alarm. We’re running out of words to describe what we actually see, what’s happening on the ground, and that’s why there’s a sense of desperation.”

But for the United Nations, which deploys aid workers to help populations hit by the shelling, “today was a lighter day,” he added.