Australian Special Forces in Iraq are working with an elite Iraqi security force accused of killing prisoners and other human rights violations.

Prime Minister Tony Abbott has confirmed that the 200-strong Australian Special Operations Task Group in Iraq has begun providing "training and assistance" for the Iraqi Counter-Terrorism Service (CTS) in its battle against Islamic State.

Military experts regard the service as the most capable and resilient element of the Iraqi security forces. However, former Australian defence intelligence officers say the service has "unquestionably been responsible for major war crimes and unnecessary civilian casualties".

Created and trained by the United States military to hunt down al-Qaeda in Iraq (AQI) militants, the service is directly responsible to Iraqi Prime Minister Haider al-Abadi and operates independently from the Iraqi defence and interior ministries.

The CTS grew rapidly under former Iraqi prime minister Nouri al-Maliki, from 1500 personnel in 2007 to more than 12,000 in early 2014. Two brigades of the Iraqi Special Operations Force (ISOF) provide the CTS's primary combat capability.