The reported assaults raise concerns that the militants are attempting to hone their chemical weapons capabilities as they push to seize more ground.

The Islamic State group's reported chlorine attacks appear to have been largely ineffectual. The attack on the police officers last month is the only one officially documented.

Chlorine, a common component in industry, is sold legally, but its use as a weapon violates the Chemical Weapons Convention.

It was on September 15 when the 11 police officers were rushed into the hospital emergency room in Balad. Some struggled to stand; tears streamed from their eyes.

"They were panicked; we were panicked," Kasim Hatim, the hospital's director, said. "We initially thought it might be a more serious gas, a nerve gas or an organophosphate."

The men had been brought in from the front lines of the nearby town of Duluiyah.

Suddenly there was a boom in the area that the extremists had just vacated, said Lieutenant Khairalla al-Jabbouri, 31, one of the survivors. "It was a strange explosion. We saw a yellow smoke in the sky," he said.

The wind carried the fog towards their lines. The men say it hung close to the ground, consistent with the properties of chlorine gas, which is heavier than air. "I felt suffocated," Jabbouri recalled. "I was throwing up and couldn't breathe."

Chlorine bombs are an easy-to-create but inexact weapon, experts say. All that is needed is a small explosive charge to rupture containers filled with the substance.

Ammar Toma, a member of parliament's defence and security committee, said it had received photographs from officers of the projectiles used in the Saqlawiyah area.

"We aren't sure chlorine was used," he said. "But we believe it was either chlorine or another chemical."