At least 30 people were killed when a Shi'ite Muslim militia opened fired inside an Iraqi Sunni mosque in the country's eastern Diyala province on Friday, an Iraqi security source said.
The security source said at least 30 bodies had arrived at the hospital in the city of Baquba in Diyala province. Witnesses said the death toll from the attack was higher, but it was not immediately possible to verify the reports.
Such sectarian violence could hurt efforts by Iraq's new prime minister, moderate Shi'ite Haider al-Abadi, to form a government that can unite Iraqis against Islamic State, the Sunni militants who have seized large parts of the country.
Ambulances transported the bodies to the town of Baquba, the main town in Diyala province, where Iranian-trained Shi'ite militias are powerful and act with impunity.
Attacks on mosques are acutely sensitive and have in the past unleashed a deadly series of revenge killings and counter attacks in Iraq, where violence has returned to the levels of 2006-2007, the peak of a sectarian civil war.
Iraqi Shi'ite militia forces executed 15 Sunni Muslims and then hung them from electricity poles in a public square in Baquba in July, police said.
Diyala police officials told Reuters they had provided Shi'ite militias with names for hit lists so that suspected members of Islamic State could be tracked and executed.