Shiite militiamen gunned down 70 people in an apparent revenge attack at an Iraqi Sunni mosque Friday, dealing a blow to government efforts to regain territory seized by jihadist-led militants.
The shooting in the Hamreen area of Diyala province will increase already significant anger among Iraq's Sunni Arab minority with the Shiite-led government, undermining an anti-militant drive that ultimately requires Sunni cooperation to succeed.
It came as government forces battled to regain ground in Diyala, and Washington warned of the dangers of the Islamic State (IS) jihadist group, which spearheaded an offensive that overran swathes of Iraq in June, and this week released a video of American journalist James Foley being beheaded.
Army and police officers said the attack on the Musab bin Omair Mosque came after Shiite militiamen were killed in clashes in the area, 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 machinegun fire, at 70 dead and 20 wounded.
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.
Elsewhere in Diyala, Kurdish and federal security forces on Friday launched an operation aimed at retaking the Jalawla area from militants who seized it on August 11.
Federal forces backed by air support also clashed with militants in the Saadiyah area south of Jalawla, officers said.
Pentagon chiefs, meanwhile, warned of the dangers of IS and said operations against it in Syria may be needed, as the West reeled from Foley's grisly killing.
"They marry ideology and a sophistication of strategic and tactical military prowess," US Defence Secretary Chuck Hagel said of the "barbaric" militants.
"They are tremendously well funded. This is beyond anything we have seen."
General Martin Dempsey, chairman of the Joint Chiefs of Staff, said the group "has an apocalyptic end of days strategic vision that will eventually have to be defeated".
A 'very long contest'
Dempsey warned the jihadist vision of a wider Muslim caliphate could "fundamentally alter the face of the Middle East and create a security environment that would certainly threaten us in many ways".
"Can they be defeated without addressing that part of the organisation that resides in Syria? The answer is no," he said, when asked if the campaign against the group could go beyond Iraq.
He spoke of a "very long contest" that could not be won by US military prowess alone, but only with regional support and that of "the 20 million disenfranchised Sunnis that happen to reside between Damascus and Baghdad".
He was referring to the alienation of many Sunni Arab Muslims from Iraq's government and the Alawite-dominated regime in Syria.
The US military said it had conducted 90 air strikes in Iraq since August 8, more than half in support of Kurdish forces near the country's largest dam on the Tigris river north of militant-held second city Mosul.
Foley's killing has stoked Western fears that the territory the militants have seized in Syria and northern Iraq could become a launchpad for a new round of global terror attacks.
The US State Department said it estimated there were about 12,000 foreign fighters from at least 50 countries in Syria.
'We don't pay ransoms'
Foley, a 40-year-old freelance journalist, was kidnapped in northern Syria in November 2012. His employer GlobalPost said his captors had demanded a 100-million-euro ($132-million) ransom.
GlobalPost CEO Philip Balboni said his team had never taken the demand seriously, and US State Department deputy spokeswoman Marie Harf insisted: "We do not pay ransoms."
His captors had also sent Foley's family a taunting and rambling email threatening to kill him.
GlobalPost released the text, which claimed that "other governments" had accepted "cash transactions" to free hostages, and that the militants had offered prisoner exchanges for Foley's freedom.
Harf said Washington estimates that IS has received millions of dollars in ransom money this year alone. Most are believed to have been paid by European governments.
In the execution video, released online, a black-clad militant said Foley was killed to avenge US air strikes against IS.
The man, speaking with a clear south London accent, paraded a second US reporter, Steven Sotloff, in front of the camera and said he too would die unless President Barack Obama changes course.
Sotloff is a freelance journalist for Time magazine who was captured on August 4, 2013.
After overrunning large areas of Iraq in June, the jihadists went back on the offensive earlier this month, attacking mainly Christian and Yazidi Kurdish areas east and west of Mosul, triggering a new exodus of civilians that helped prompt the US air strikes.