Two Iraqi militias linked to the Kurdistan Workers' Party (PKK) are recruiting child soldiers, Human Rights Watch said on Thursday.

The rights group said it had documented 29 cases of Kurdish and Yezidi children recruited by the People's Defence Forces (HPG) -- an armed wing of the PKK -- and the Shingal Resistance Units (YBS).

The HPG has forces in Turkey, Syria and Iraq, and is fighting against Turkish forces and also against militias including the Islamic State group.

"In two cases the armed groups abducted or seriously abused children who tried to leave their forces," HRW said in a statement.

"The groups should urgently demobilise children, investigate abuses, pledge to end child recruitment, and appropriately penalise commanders who fail to do so."

The PKK is a Kurdish separatist group which Turkey considers to be a "terrorist" organisation.

The YBS is formed largely of fighters from the Yezidi religious minority, which has faced a campaign of extermination by IS.

The jihadist group sees Yezidis as infidels and massacred Yezidi civilians in Sinjar in August 2014, executing men and abducting women as sex slaves.

HRW said some of the children it interviewed had taken part in fighting, while others had manned checkpoints or cleaned and prepared arms.

One, a young Yezidi at a refugee camp in Iraqi Kurdistan, said he had joined the HPG in 2014 when he was 14 and fought in Syria until July 2016.

"Even if the armed groups do not send children into direct combat, they place them at risk by training them in areas that Turkey has attacked with air strikes in its conflict with the PKK, such as Iraq's Qandil mountain area," HRW said.

Under international law, the recruitment or use of children under 15 as fighters is a war crime.