Isis now has chemical weapons, the US military has confirmed.
A rocket launched by militants at American troops in Iraq contained a mustard agent.
However the bomb was not very advanced, and so no one was hurt in the attack on the Qayyarah air base near Mosul on Tuesday.
Marine Gen Joseph Dunford, chairman of US Joint Chiefs of Staff, called the incident a ‘concerning development’.
US officials had said on Wednesday that an oily substance found on a fragment of the rocket that landed inside the security perimeter of the base had initially tested
positive for mustard agent.
A second test was negative, however. Further lab testing has been scheduled.
‘We assess it to be a sulphur-mustard blister agent,’ Dunford said, before adding that the rocket appeared rather rudimentary.
He said that while Isis has a ‘chemical warfare network’, its means of making effective chemical weapons are limited.