Ammon News - AMMONNEWS - -Daesh terror 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 late Monday.

Pentagon spokesman,Navy Captain Jeff Davis, remarks came a week after a rocket with a possible chemical agent landed near U.S. troops.

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 Davis told reporters.

The rocket fired last 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.

"We fully recognize this is something that Daesh has done before. They've done it many times, at least a couple dozen that we know of where they have launched crude makeshift munitions that are filled with this mustard agent," Davis said .

An air strike by the U.S.-led military coalition destroyed a Daesh chemical weapons factory on Friday near Qayyara, the second attack against a chemical arms facility this month.

Davis added that Daesh ability to use mustard gas as weapon has been rudimentary. The group typically uses a chemical powder bound together with oil, which leaves behind a telltale oil trace.