"A mass grave was discovered on Thursday in the Duraji area" where there are many caves once used as hideouts by the Islamic State group (IS), the official said.

Duraji is in Salaheddin province in an area disputed between the federal government and the autonomous region of Kurdistan.

"The bodies of at least 11 Iraqi police officers have been taken from the grave so far" since the operation began in the morning, said the official.

"We think they had been prisoners of IS in 2018," the official said, adding that both peshmerga and Iraqi federal police were taking part in the search.

He said the mass grave was discovered as a result of "information obtained from IS hideouts in the region where the jihadists imprisoned members of the Iraqi forces they captured".

IS seized swathes of Iraq in a lightning offensive in 2014, before being beaten back by a counter-insurgency campaign supported by a US-led military coalition.

The Baghdad government declared the Sunni extremists defeated in late 2017, although IS retains sleeper cells which still strike security forces with hit-and-run attacks.

On December 5, four peshmerga fighters were killed and five wounded in an attack on an outpost north of Kirkuk that officials blamed on IS.