The abduction and killing of scores of Sunni civilians in eastern Iraq this month and attacks on their property by Iranian-backed Shiite militiamen could constitute a war crime, Human Rights Watch (HRW) said Sunday.

Shiite militiamen deployed this month in Muqdadiya, 80 km (50 miles) northeast of Baghdad, after two blasts killed 23 people near a coffee shop where they often meet. The Islamic State group claimed responsibility for the attacks, which it said had targeted Shiites.

New York-based HRW said members of the Badr Organisation and Asaib Ahl al-Haq, leading groups in the government-run Popular Mobilization Forces fighting ISIS, were responsible for retaliatory attacks it described as "serious violations of international humanitarian law."

"Again, civilians are paying the price for Iraq's failure to rein in the out-of-control militias," said Joe Stork, deputy Middle East director at HRW.

"Countries that support Iraqi security forces and the Popular Mobilization Forces should insist that Baghdad bring an end to this deadly abuse."

A U.S.-led coalition comprised of European and Arab powers has carried out thousands of airstrikes against ISIS targets in the past 18 months and provided training and assistance to Iraqi security forces. Western states have also given Baghdad financial aid.

The rise of the Islamist militant group Islamic State, which follows a Sunni jihadi ideology, has exacerbated a long-running sectarian conflict in Iraq, mostly between the Shiite majority and minority Sunnis.

A surge in such violence would represent a further challenge to Prime Minister Haider al-Abadi, a moderate Shiite Islamist who is trying to reconcile the Sunnis and win them over to push ISIS out of the mainly Sunni-populated areas in the country's north and west, which it seized in 2014.

HRW cited Sunni residents of Muqdadiya who said the militias were behind the attacks on their houses, mosques and people. Reuters could not independently verify their accounts.

Sunni lawmakers told Reuters earlier this month more than 40 people had been killed and at least nine mosques fire bombed in Diyala, the province where Muqdadiya is located.

The militias rejected the casualty figures and said the attacks on Sunnis aimed to stir sectarian tension in Diyala, which lies between Baghdad and the Iranian border and has a mixed population of Shi'ites and Sunnis.

Iraqi officials declared victory over the insurgents in Diyala nearly a year ago, but ISIS has remained active. Badr has established itself as the ascendant militia in the region after rolling back the militants.