The device was "imprecise and crude," a Pentagon spokesperson told the DPA news agency on Wednesday, commenting on the attack on the Qayara West air base in Iraq.

The missile landed inside the perimeter during an "indirect fire" attack from the "Islamic State" (IS), another Pentagon official told reporters. "Hundreds" of US troops were located in the Iraqi compound, he added.

An initial test of the shell fragments showed presence of mustard gas residue. The second probe turned out negative and the samples have been sent to laboratory for further testing.

The source said that nobody at the base was injured.

"Our concern is not much greater after seeing this," said the official, who asked not to named.

Bracing for push on Mosul

After the attack, a small group of US troops inspected the shell fragments and discovered a black oily substance on one of them, which tested positive for mustard gas. Although none of the soldiers displayed symptoms of exposure, they all went through the standard decontamination procedure.

"This attack has not impacted our mission in any way, nor have we changed our security posture in the area around Qayara," the Pentagon said in a statement. "We train and equip ourselves and our partners for just this type of eventuality."

The US soldiers are helping the Iraqi forces prepare the upcoming offensive to nearby Mosul, which is the last major IS stronghold in Iraq.

A UN probe has found that IS has used toxic gas in Syria. Last week, the US-led coalition bombed a factory near Mosul, allegedly used by the militia to produce chemical weapons.

dj/kl (dpa, AP, Reuters, AFP)