Islamic State militants are “dead set” on using chemical arms and are likely to try them again as Iraqi forces advance on Mosul, a Pentagon spokesman said on Monday, a week after a rocket with a possible chemical agent landed near U.S. troops.

The rocket fired Tuesday landed in an unpopulated area near Qayyara West base, several hundred yards from where hundreds of U.S. troops are working to prepare an airfield for an Iraqi offensive to recapture the city of Mosul. No one was hurt in the attack.

The shell initially tested positive for a mustard agent, but two subsequent tests have been inconclusive and the device is undergoing further tests, Navy Captain Jeff Davis, a Pentagon spokesman, told reporters. […]