BAGHDAD (Reuters) - Iraqi military commanders should prevent militias with records of serious abuses from taking part in a planned offensive on the Islamic State-held city of Mosul, campaign group Human Rights Watch said on Sunday.

The battle for Mosul, the ultra-hardline militants’ de facto capital in Iraq and the largest city anywhere in their self-proclaimed caliphate, is expected later this year but plans have not been finalised, officials and diplomats in Baghdad say.

Army, police and counter-terrorism forces are expected to participate, backed by air support from a U.S.-led coalition.

The role of Kurdish peshmerga forces and Shi’ite Muslim militias from the Popular Mobilisation Forces (PMF) is unresolved and remains a point of contention. They will likely join in the larger battle but may be restricted to the city’s outskirts, the officials say.

“Iraqi commanders shouldn’t risk exposing Mosul civilians to serious harm by militias with a record of recent abuse,” said Joe Stork, deputy Middle East director at Human Rights Watch, echoing the positions of many Western diplomats and aid workers who are mobilising humanitarian aid for the population.

Shi’ite militias and peshmerga fighters have been key forces in Iraq’s campaign to retake the third of the country seized by Islamic State in 2014 after army and police units collapsed, but they have also been accused of abuses against civilians, allegations they deny or dismiss as isolated cases.

Their participation in the battle for Mosul, a predominately Sunni Arab city which also has diverse ethnic and sectarian communities, risks confrontation with the local population.

Militia leaders say the security forces have not been rebuilt enough to retake the city by themselves in a battle that could see fierce street fighting.

Mosul officials, displaced to other parts of the country, say alleged abuses in Falluja in May, alongside those in previous battles, vindicate their calls to keep the militias out of the northern city.

Several militias faced allegations in Falluja from the provincial governor, which they denied, that they executed 49 Sunni men and detained more than 600 others. The authorities opened an investigation and made several arrests at the time.