The Iraqi military deliver aid and evacuate vulnerable residents from the town of Amerli in northern Iraq, which has been besieged by Islamic State militants for the past two months. The U.N.’s representative in Iraq, Nickolay Mladenov, expressed alarm on the situation and warned of a possible massacre should the Islamist militants overrun the town. (Iraqi MOD via YouTube)

Suspected Shiite militiamen fired on a Sunni mosque in eastern Iraq on Friday, killing dozens of worshipers and stirring fears of an explosion of sectarian violence resulting from the rise of the Islamic State extremist group.

The gunmen stormed Mosab Bin Umar mosque in Diyala province during Friday noon prayers. News services put the death toll at more than 60; area health officials and security forces offered figures ranging from 35 to 70.

It was the single deadliest assault in months on Sunni civilians in Iraq. The attack highlighted the increasing danger that Iraq could slide into sectarian war as hard-line Shiite militias, which are often stronger than the country’s army, take the lead in responding to violence by Islamic State fighters and other Sunni militants.

The country’s newly designated prime minister, Haider al-Abadi, is under pressure to form a government capable of confronting the Islamic State and uniting the fractured country.

But Friday’s attack could complicate that process. Sunni lawmakers from Diyala blamed the killings on Shiite militias tied to the government. And the Sunni party of Iraq’s new parliamentary speaker, Salim al-Jubouri, said it would boycott Abadi’s government, Jubouri’s spokesman said.

Iraqi and Kurdish forces are battling Islamic State fighters to retake two towns in northern Iraq, after their strategic win of recapturing the Mosul Dam. (Reuters)

The Islamic State gained control of much of northern Iraq in June and declared its own caliphate, or state, spanning portions of Iraq and Syria. The extremists are believed to have executed thousands of Shiites and political opponents in their push to impose a radical interpretation of Islamic law.

The U.S. military has carried out dozens of airstrikes against Islamic State targets over the past two weeks, with the Obama administration saying it wanted to protect U.S. citizens and critical infrastructure and prevent a genocide of the Yazidis, a religious minority.

The U.S. State Department condemned Friday’s mosque attack in the village of Imam Wais. State Department deputy spokeswoman Marie Harf said it “underscores the urgent need for Iraqi leaders from across the political spectrum to take the necessary steps that will help unify the country against all violent extremist groups.”

Saad Maan, a spokesman for Iraq’s Interior Ministry, said the attack followed a bombing less than a mile away that targeted a gathering of security forces and Shiite militiamen.

Thousands of young Shiite men flocked to join militias and the armed forces after the Islamic State’s initial offensive in June. On Friday, the parliament speaker’s spokesman, Ahmed al-Jubouri, suggested that the militias were departing from their initially stated mission. “The volunteers are supposed to help the army defeat the Islamic State,” he said. Instead they have “taken advantage” of their empowerment to attack Sunnis, he said.

Shiite militia is mobilized

Meanwhile, the leader of a powerful Shiite militia, the Badr Brigades, mobilized his fighters Friday to liberate the northern Shiite town of Amerli, which has been besieged by Islamic State militants.

Residents of Amerli, 120 miles north of Baghdad, have called for U.S. intervention to save them from extermination. This month, the American military used airstrikes to open a humanitarian corridor for thousands of Yazidis. They had fled their hometown ahead of advancing Islamic State fighters and ended up stranded on Iraq’s northwestern Mount Sinjar.

Amerli’s residents and political leaders say that at least 13,000 people, Shiite members of Iraq’s ethnic Turkmen minority, have been trapped in the town since June.

Shiite activists argue that Amerli is another Mount Sinjar. The difference, said Fawzy Akram, a Turkmen Shiite lawmaker from the town, is that “they’re Shiites.”

The Americans “only care about Kurds, Yazidis and Christians,” said Abdel Karim al-Ansari, a lawmaker from the Badr organization, the group that sent its militia to free Amerli.

A State Department official in Washington said Friday that American officials felt “grave concern” about the humanitarian crisis in Amerli and other parts of northern and central Iraq.

The U.S. government is “working through the U.N. and other humanitarian organizations to provide much-needed aid,” in coordination with the Iraqi government, the official said, speaking on condition of anonymity because of the sensitivity of the situation. The official added that American authorities are reviewing options with the Iraqi government “as it is clear that much more needs to be done. Up to 1.4 million people in Iraq have been displaced by [the Islamic State’s] violence, and many more are still at risk.”

The siege of Amerli

Hadi al-Ameri, who is Iraq’s transport minister as well as the head of the Badr organization and its militia, moved 2,000 fighters to a town near Amerli on Friday, members of his political bloc said.

The Shiite militia has tried before and failed to break the siege of Amerli, but this deployment is the largest so far, said Ansari.

“I have more faith in Hadi al-Ameri because the Iraqi army is disorganized and retreats every time,” Mehdi al-Bayati, a high school principal-turned-activist in the town, said by phone.

Residents of Amerli say they lack food, water, medicine and electricity. According to Bayati, 10 children have died in 10 days.

Several government airdrops of aid have done little to alleviate the suffering, Bayati said.

Wayne Hsieh, an assistant professor of military history at the U.S. Naval Academy who worked near Amerli as a U.S. government official in 2009, speculated that it was easier for the U.S. government to help the Yazidis because they fall outside Iraq’s escalating Sunni-Shiite conflict. The Yazidis follow a faith that draws on various pre-Islamic and Persian traditions.

“There is always the risk: We break the siege and the Shiite militias go out of control,” Hsieh said, referring to the possibility that Shiite militias could use a humanitarian corridor to Amerli to gain access to nearby Sunni towns and exact revenge.

“But if the Islamic State takes the town, that is a guaranteed catastrophe,” he said.

Anne Gearan in Washington and Mustafa Salim in Baghdad contributed to this report.