Baghdad, Iraq (CNN) -- Suspected Shiite militiamen opened fire Friday inside a Sunni mosque in northeast Iraq, killing dozens in an attack that appeared to derail the formation of a new government -- something world leaders have said is a must if the country hopes to defeat Islamic militants.
The news of the attack that left 65 people dead came as Iraqi forces -- in some cases boosted by U.S. airstrikes -- touted successes in the fight to push back the extremist Sunni Muslim militant group, which calls itself the Islamic State. Militants control large swaths of northern Iraq and eastern Syria.
The attack in the Musab bin Omar Mosque in the village of Bani Weis in Diyala province, about 75 kilometers northeast of Baquba, also left at least 17 people wounded, officials said.
Sunni lawmakers withdrew from negotiations to form a new government, saying they would not return until those behind the attack were arrested, two party officials told CNN.
Iraqi authorities did not immediately identify the attackers, but Sunni politicians have put the blame on Shiite militias.
Calling it the latest in a series of attacks by militiamen aligned with Iraqi security forces in the fight against ISIS, Deputy Prime Minister Saleh al-Mutlaq -- a Sunni -- said the government must conduct an investigation and hold accountable those behind the attack.
"We want serious action," he told CNN.
But the United States and the United Nations called on Iraq to continue its efforts to form a united government, saying it is the only way for Iraq to defeat the sectarian divide that allowed ISIS to take hold.
"This senseless attack 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," Marie Harf, a U.S. State Department spokeswoman, said.
Sunni lawmaker Nahida al-Dayani, who was grew up in the village, told Reuters that roughly 150 people were worshipping inside the mosque Friday morning when Shiite militiamen stormed the building.
The attack, al-Dayani told the news agency, was believed to be retribution for a roadside bombing earlier in the day that targeted an Iraqi security force vehicle.
U.S. 'understands the threat'
U.S. President Barack Obama authorized targeted airstrikes to protect U.S. personnel in Iraq -- including military advisers -- as well as minorities being brutalized by ISIS. The Pentagon also has said the airstrikes are meant to support Iraqi and Kurdish defense operations.
Much has been made in recent days about whether the United States needs to expand its operations to go after ISIS fighters in their stronghold in eastern Syria.
Asked Friday whether the United States was considering expanding the air campaign into Syria, Pentagon spokesman Rear Adm. John Kirby said, "We don't telegraph our punches."
"I'm not going to get ahead of planning that hasn't been done or decisions that haven't been made," Kirby said. "... I think you can rest assured that the leadership here in the Pentagon understands the threat posed by this group, understands the threat posed inside Iraq."
ISIS vows retribution for killings
ISIS, through one of its Twitter accounts, said Friday that more than 50 of its fighters were killed while defending Jalawla "against the Peshmerga mercenaries," a reference to Kurdish forces.
A subsequent tweet said the group vowed revenge.
The threat came as Iraqi and Kurdish forces lauded gains against ISIS fighters, taking back several villages in Diyala province, and they surrounded the large ISIS-held town of Jalawla, a spokesman for Kurdish forces said.
The Peshmerga and Iraqi commandos inflicted heavy losses against ISIS on Friday as they took back a number of towns and villages around Jalawla in Diyala province, Pershmerga spokesman Brig. Gen. Halgord Hikmat told CNN.
Jalawla, a mostly Kurdish city of about 50,000 people roughly 70 miles northeast of Baghdad, was taken by ISIS earlier this year. Kurdish and Iraqi forces now surround it, Hikmat said.
In neighboring Salaheddin province, also north of Baghdad, Iraqi helicopters killed 30 ISIS fighters in the town of Dhuluiya, about 70 kilometers northeast of Baquba, Iraqi security officials told CNN.
Also Friday, Iraqi forces took another step in their attempt to take back Tikrit, the birthplace of former Iraqi ruler Saddam Hussein, Iraqi security sources said. The city fell to ISIS in June.
More U.S. airstrikes near Mosul Dam
Meanwhile, U.S. aircraft conducted three more airstrikes against the militants, primarily ISIS-driven vehicles, near the Mosul Dam, the Pentagon said.
Since August 8, the U.S. military has carried out 93 airstrikes, 60 of them in support of Iraqi forces near the Mosul Dam, according to the Defense Department.
The dam is the largest in the country, and Iraqi and U.S. officials fear that a breach in the dam would threaten the lives of millions of Iraqis who live downstream in Mosul and Baghdad.
The U.S. airstrikes have continued despite ISIS' threat to kill another American hostage.
Earlier this week, ISIS posted to the Internet a video showing the execution of American journalist James Foley. The group threatened the life of another man, believed to be American journalist Steven Sotloff, if the United States didn't end airstrikes in Iraq.
Obama vowed justice in a televised address on Wednesday, saying the country would "continue to do what we must do to protect our people."
CNN's Jomana Karadsheh reported from Baghdad, and Jason Hanna and Chelsea J. Carter reported and wrote from Atlanta. CNN's Mohammed Tawfeeq and Hamdi Alkhshali contributed to this report.