Washington has declared the beheading of an American journalist a "terrorist attack", upping the stakes in its confrontation with the Islamic State (IS) jihadist group, as Shiite militiamen gunned down 70 people at an Iraqi Sunni mosque.
The apparent revenge attack at the mosque in Diyala province yesterday will increase already significant anger among Iraq's Sunni Arab minority with the Shiite-led government, undermining an anti-militant drive that requires Sunni cooperation to succeed.
It came as the US, which is carrying out airstrikes against IS, ramped up its rhetoric over the grisly killing of journalist James Foley, which was carried out by the jihadist group and shown in a video posted online.
In Washington, Deputy National Security Advisor Ben Rhodes said the beheading of Foley "represents a terrorist attack against our country".
Rhodes also said that paying ransoms to free hostages is "not the right policy", confirming Washington's long-standing position amid claims from IS that other countries had paid to have their nationals freed.
In an unanimous statement yesterday, the UN Security Council strongly condemned Foley's murder as "heinous and cowardly".
Army and police officers said the attack on the Musab bin Omair Mosque in Diyala yesterday came after Shiite militiamen were killed in clashes, while other sources said it followed a roadside bomb near one of their patrols.
Doctors and the officers put the toll from the attack, in which worshippers were sprayed with machine gun fire, at 70 dead and 20 wounded.
Two officers had earlier blamed IS for the attack, but the preponderance of accounts point to Shiite militiamen.
The government turned to militiamen to bolster its flagging forces during the IS offensive, sparking a resurgence of groups involved in brutal sectarian killings in past years that will be difficult to dislodge.
Ibrahim Aziz Ali, whose 25-year-old nephew was among those killed, told AFP he and other residents heard gunfire and rushed to the mosque, where they were fired on by snipers.
"We found a massacre" at the mosque, he said.
Five vehicles with images of revered Shiite Imam Hussein were parked at the mosque, Ali said, adding that residents clashed with the militiamen who withdrew when the Iraqi army arrived.
Iraqi premier designate Haidar al-Abadi issued a statement calling for unity and condemning the killings, which may complicate the already-contentious process of forming the country's next government.