Forces loyal to the Government of Iraq have been reinforcing positions around the town of Samarra against the Islamist insurgents who have taken control of large parts of the country.
Government troops have been joined by militia formed after appeals by Prime Minister Nouri al-Maliki and a leading Shia cleric.
The call by a representative of Grand Ayatollah Ali al-Sistani came as the militants seized more towns and widened their grip in the north and east.
Led by the Islamic State in Iraq and the Levant (ISIS), the Sunni insurgents have threatened to push towards the capital and other regions dominated by Iraq's Shia Muslim majority, whom they regard as "infidels".
American President Barack Obama has promised action against the insurgents, but has ruled out putting American troops on the ground.
Mr Obama said he will make a decision in the "days ahead" about how to support Iraqi forces confronting the Sunni insurgents.
The president said Islamic extremism in the country posed a threat to American interests as well, but a short-term military response would not help.
He said America had invested heavily in strengthening Iraq's domestic security forces, and their failure to stand their ground against the jihadists reflected the political problems in the country.
Mr Obama has not ruled out air strikes, but said any such intervention must be met by a plan for long-term stability from the Iraqis.
American troops last withdrew from Iraq in 2011, after invading in 2003.
The taking of Mosul
The United Nations said the number of people killed after Sunni Islamist militants overran the Iraqi city of Mosul earlier this week may run into the hundreds.
Human rights spokesman Rupert Colville said his office had reports the killings included the execution of 17 civilians working for the police and a court employee in central Mosul.
Four women had killed themselves after being raped, 16 Jordanians had been kidnapped, and prisoners released by the militants had been looking to exact revenge on those responsible for their incarceration.
The UN has also had reports suggesting that government forces have also committed excesses, in particular the shelling of civilian areas.