An attack by suspected Shia militiamen on a Sunni mosque in Iraq's Diyala province has killed at least 68 people.
Officials say a bomber blew himself up in the mosque during Friday prayers and gunmen fired on fleeing worshippers.
Diyala province has seen heavy fighting in recent weeks between IS and Iraqi troops backed by Shia militiamen.
The attack is seen as a blow to government efforts to secure backing from Sunni groups in its battle against Islamic State (IS) jihadists.
Prime Minister designate Haider al-Abadi, a moderate Shia, is trying to form a more inclusive government - following international criticism of outgoing PM Nouri Maliki, who was widely seen as a divisive figure.
But two influential Sunni politicians, Parliamentary Speaker Salim al-Jabouri and Deputy Prime Minister Saleh al-Mutlak, have suspended participation in talks on a new cabinet in protest at Friday's massacre.
It took place in a village mosque south of the city of Baquba, about 120km (75 miles) from Baghdad.
A security official told AFP news agency that Shia fighters allied with the government had carried out the assault as a reprisal for a bombing that had targeted their fighters.
Jim Muir, BBC News, Iraq
It's just the kind of sectarian provocation that is threatening to plunge the country back into the dark days of 2006 and 2007, when many thousands died in a vicious spiral of communal carnage.
It comes at a delicate moment, when the Shia Prime Minister designate, Haider al-Abadi, is trying to persuade the alienated Sunnis to take part in a broad, inclusive new government, hoping to turn them against the militants of the Islamic State.
Bringing the Sunnis on board is a key element in the American strategy for isolating the Islamic State militants, who have capitalised on Iraqi Sunni grievances.
But actions like the latest massacre in the mosque are not going to make that task any easier.
IS has seized large swathes of Iraq and Syria in recent months. Since 8 August the US has backed Iraqi and Kurdish troops tackling the insurgents by conducting air strikes.
On Thursday, US Defence Secretary Chuck Hagel described the group as an imminent threat to the US.
"They marry ideology, a sophistication of strategic and tactical military prowess, they are tremendously well-funded. This is beyond anything that we have seen."
Gen Martin Dempsey, the chairman of the US Joint Chiefs of Staff, said IS was "an organisation that has an apocalyptic, end-of-days strategic vision and which will eventually have to be defeated".
He also said that IS fighters could not be defeated without attacking their bases in Syria. The militants, he said, should be confronted "on both sides of what is essentially at this point a non-existent border".
Hunt for killer
Meanwhile Britain has said it will not work with Syrian President Bashar al-Assad in the battle against IS, despite suggestions from a retired top general that it should do so.
The warnings came after IS beheaded US journalist James Foley.
UK security services are trying to identify the jihadist who appeared in footage of his killing. Unconfirmed reports suggest the man - who had an English accent - is from London or south-east England.
In the video of Mr Foley's murder, IS militants threatened to kill another American if the US did not stop its air strikes against the group in northern Iraq.
- Formed out of al-Qaeda in Iraq (AQI) in 2013, IS first captured Raqqa in eastern Syria
- By early 2014 it controlled Falluja in western Iraq
- Has since captured broad swathes of Iraq, seizing the northern city of Mosul in June
- Fighting has displaced at least 1.2 million Iraqis
- Pursuing an extreme form of Sunni Islam, IS has persecuted non-Muslims such as Yazidis and Christians, as well as Shia Muslims, whom it regards as heretics
- In July alone, IS expanded dramatically, recruiting some 6,300 new fighters largely in Raqqa, an activist monitoring group said