The US military has concluded that a rocket fired earlier this month at an Iraqi air base housing hundreds of US troops contained no mustard gas, as initially suspected.

Test results released Tuesday "determined there were no chemical warfare agents present in the munitions fired toward the Qayyarah-West Airfield, Iraq, Sept 20, 2016," according to the US military command in Baghdad.

No one was injured in the rocket attack and no one showed any immediate signs of exposure to the suspected mustard agent.

But a field test of residue from fragments recovered from an impact site had indicated the possible presence of mustard gas.

That prompted further tests by two separate laboratories, which confirmed that "no chemical warfare agent were present in the munitions," the statement said.

The Pentagon has expressed concern that Islamic State militants could use mustard gas to defend their stronghold at Mosul, Iraq's second largest city, against an Iraqi offensive.

Pentagon spokesman Jeff Davis told reporters on Monday that jihadists have already used a rudimentary form of mustard gas at least a couple of dozen times.

Hundreds of US troops are based at Qayyarah-West, which is shaping up as a major logistics hub for the Mosul offensive.

A US official said Wednesday that Washington was prepared to send more troops to Iraq to train and advise Iraqis as planning intensifies for the Mosul offensive.

Mustard gas was first used by the German army in 1917. It causes large blisters on exposed skin and attacks the eyes and lungs. Its use was banned in 1925 under the Geneva Protocol and later by the Chemical Weapons Convention in 1993.