Iraq's most senior Shia cleric has issued a call to arms as Sunni-led insurgents sweep across towns.
The call by a representative of Grand Ayatollah Ali al-Sistani came during Friday prayers, as the militants widened their grip in the north and east and threatened to march south.
The UN says hundreds have been killed - with militants carrying out summary executions of civilians in Mosul. The US and Iran have promised to help the fight against the insurgency.
Led by the Islamic State in Iraq and the Levant (ISIS), the insurgents have threatened to push to the capital, Baghdad and regions further south dominated by Iraq's Shia Muslim majority, including the holy cities of Najaf and Karbala, whom they regard as "infidels".
In his sermon at Friday prayers in Karbala, Sheik Abdulmehdi al-Karbalai said: "Citizens who are able to bear arms and fight terrorists, defending their country and their people and their holy places, should volunteer and join the security forces to achieve this holy purpose."
President Hassan Rouhani of Iran called Iraqi Prime Minister Nouri Maliki on Thursday and promised that Iran would "not allow the supporters of terrorists to disrupt security and stability of Iraq through exporting terrorism to Iraq".
According to the Wall Street Journal - which cited unnamed sources - Iran has already deployed two battalions of the elite al-Quds forces of its Revolutionary Guard to help the Iraqi government.
Rape and murder UN human rights spokesman Rupert Colville said on Friday that his office had verified reports that included the killing of 17 civilians working for the police and 12 Iraqi soldiers.
Among the atrocities, he listed four women who had killed themselves after being raped. He said there had been government "excesses", too, and cited the shelling of civilian areas on 6 and 8 June.
"There are claims that up to 30 civilians may have been killed," Colville said.
The United Nations refugee agency, UNHCR, says local authorities estimate that up to 300,000 people fled Mosul in the past few days - joining the more than 500,000 displaced by the conflict in Anbar province earlier.
However, the number of those arriving has slowed down and some already there have begun to return. In the north of the country, Kurdish forces have claimed control of the oil city of Kirkuk, saying government forces have fled.
The Kurds - seen as a bulwark against the Sunni Muslim insurgents - have also been locked for years in a dispute with Baghdad over Kirkuk, seeking to incorporate it into their own autonomous area