Human Rights Watch said Sunday that powerful Iraqi Shiite militias were behind revenge attacks against Sunnis earlier this month that erupted after the Islamic State group bombed a cafe frequented by militiamen.

The New York-based rights group issued a report saying the Badr Brigades and Asaib Ahl al-Haq, two Iran-backed militias, carried out the Jan. 11 retaliation attacks in the town of Muqdadiyah, northeast of Baghdad in the mixed Diyala province. The attacks came after a double suicide bombing at the cafe killed at least 32 people.

Citing unnamed residents, HRW said the militiamen killed at least a dozen people and demolished Sunni mosques, homes and shops. It described the attacks as "heinous" and called for the prosecution of those responsible.

Spokesmen for the militias, who have denied previous accusations of wrongdoing, could not immediately be reached.

Shiite militias led the fight against IS in Diyala and took over much of the province's security after it was declared liberated in early 2015. Many Iraqis who initially fled IS say security concerns continue to prevent them from returning home. Sunnis make up the vast majority of those displaced by the fighting in Iraq.

The government said Sunday it needs $1.6 billion to respond to the nationwide humanitarian crisis. It said more than 3 million people have been forced from their homes by violence since IS overran the country's second largest city, Mosul, and large swaths of the north and west in the summer of 2014.

Iraq is also facing a severe financial crisis exacerbated by plunging oil prices. Oil revenue makes up nearly 95 percent of the national budget.

"What (the Iraqi government is) really saying is our back is against the wall, we need help," said Lise Grande, the U.N.'s deputy envoy to Iraq.

"We're fighting ISIL on behalf of the international community, but we just don't have the resources to look after our own people as well and we need international assistance," she said, using an acronym for IS.

Even before the IS onslaught and the plunge in oil prices, Iraqi authorities struggled to maintain aging infrastructure and provide basic services like electricity. Earlier this month, Iraq was ranked one of the 10 most corrupt countries in the world, according to Transparency International, an international monitoring group.