Iraqi government forces, backed by Shia Muslim and Kurdish militias, are reportedly holding back an advance by Sunni militants north of Baghdad.
A number of towns have been retaken from the rebels, but they still control the key cities of Tikrit and Mosul.
Reports speak of heavy clashes round the city of Tal Afar, west of Mosul, with mortar shelling of some districts.
A US aircraft carrier has been deployed to the Gulf in response to the escalating violence.
In a televised news conference, Iraqi army spokesman Lt Gen Qasim Ata, said the military had "regained the initiative", scoring successes against the militants in several areas and killing 279 of them.
However, the government's figures could not be independently verified.
In Tal Afar, inhabited mainly by Sunni and Shia Turkmens, residents told the BBC that troops clashed with militants trying to enter the city.
Government officials, quoted by AP news agency, said the rebels were using rockets captured from troops, and the local garrison had suffered heavy casualties.
The BBC's Jim Muir, reporting from northern Iraq, says government forces are building up in the city of Samarra.
They appear to be preparing for a counter-offensive on Tikrit - the hometown of Iraq's former President Saddam Hussein, who was toppled by the US invasion in 2003 - our correspondent says.
A resident of Tikrit, speaking to the BBC, said clashes were under way between militants and soldiers based at former US "Camp Speicher", north of the city.
In other developments:
• Three Iraqi soldiers and three Shia militiamen were killed in a mortar attack on a military recruitment centre near the city of Baquba, north-east of Baghdad
• In one town that changed hands, Ishaq, security forces said they had found the incinerated bodies of 12 policemen
• Seven Kurdish fighters were killed by mistake by a government helicopter in the town of Jalawla
Many Sunni rebels are fighting under the banner of the Islamic State in Iraq and the Levant (ISIS), an offshoot from al-Qaeda.
Iraq's most senior Shia cleric, Grand Ayatollah Ali al-Sistani, has issued a call to arms to fellow Shia, and there are reports that thousands have already joined militias.
Credit: The BBC