Barrel bombs, consisting of shrapnel and metal scraps packed into a barrel with explosives, are dropped routinely from helicopters over residential areas in rebel-controlled zones throughout Syria.

Losing the Hama base was a “significant risk” to the regime and U.S. analysts judged that the use of chemical weapons was “linked to a battlefield desperation decision to stop the opposition from seizing those key regime elements.” The official could not be named under the rules of the briefing.

A Syrian rebel spokesman spoke of enormous government losses on the ground since the offensive began March 21. Hours after the battle began, the U.S.-supported force captured more than 20 towns and villages, as well as 50 military outposts in the Hama countryside, said Lt. Mahmoud Mahmoud, spokesman for the Izza Army, a moderate rebel group, in a WhatsApp conversation. Regime forces “withdrew without showing any resistance,” he said, and rebels advanced to within a few miles of the military airport, which is about 50 miles northwest of Shayrat.

The surprise attack threw regime forces off-balance, and at one point regime warplanes were bombing regime positions, Mahmoud said. He claimed the regime lost more than 700 fighters. He said rebels have documented the capture or destruction of 40 regime tanks and armored personnel carriers, the destruction of 13 tanks and a great many artillery pieces.