Government forces in Iraq have used their limited air power to hit Islamist-led militants advancing from the north towards the capital Baghdad.They are fighting to push back ISIS and its allies in Diyala and Salahuddin provinces, after the militants overran the second city, Mosul, last week.
Fighting is reported in the western city of Ramadi.
Iraq's biggest oil refinery, Baiji, is reportedly under attack by mortars after being shut down on Tuesday.
Prime Minister Nouri al-Maliki appeared on television with Sunni Muslim and Kurdish leaders on Tuesday to issue a call for national unity, in the face of the advance of ISIS (Islamic State in Iraq and the Levant) and its Sunni Muslim allies. They demanded that non-state forces lay down their arms.
However, such a call is unlikely to have much effect as Mr Maliki has openly sponsored the formation of Shia Muslim militias to fight alongside regular Iraqi troops, the BBC's Jim Muir reports from Irbil in northern Iraq.
UK Prime Minister David Cameron will hold talks with his senior security advisers on Wednesday to discuss the crisis, warning that ISIS represents a "real threat" to Britain.
Unity call Militants in the western province of Anbar, where the capital is Ramadi, said they had made advances, with a number of police stations near the town of Hit going over to dissident tribes.
Using unusually strong language, Mr Maliki accused Saudi Arabia - which is largely Sunni - of backing ISIS.
He also fired four army commanders for failing to halt the sweeping advance by the militants. They included the top commander for Nineveh, the first province where ISIS fighters made major gains.
Analysis: John Simpson, BBC World Affairs Editor, Baghdad
Many Sunnis, particularly the conservative ones who started turning against al-Qaeda eight years ago, and enabled the US forces to leave Iraq with what seemed at the time to be dignity, are not at all happy that ISIS should control their towns and villages.
But the danger of the present fight-back by Shia volunteers is that they will victimise ordinary Sunnis, and make them feel that ISIS is the only group that can protect them.
In other words, this has the potential to turn into a clear-cut religious war, with the possibility of mass "cleansing" of civilians and brutality on a large scale.
Militants attacked the Baiji refinery, north of Baghdad, with mortars and machine-guns on Wednesday, officials said.
Foreign personnel were evacuated earlier but local staff reportedly remain in place, with the military defending the facility.
With Shia areas of the capital bombed almost daily, correspondents say inhabitants of Baghdad have developed a siege mentality.
People with enough money have started to stockpile essential items of food, correspondents say, which has increased prices dramatically.
In other developments:
- President Obama will brief top congressional leaders on Irag at the White House on Wednesday
- The US is reportedly considering the possibility of deploying a small number of special operations forces to Iraq to help train security forces
- Estimated 10,000 fighters in Iraq and Syria
- Joined in its offensives by other Sunni militant groups, including Saddam-era officers and soldiers, and disaffected Sunni tribal fighters
- Exploits standoff between Iraqi government and the minority Sunni Arab community, which complains that Shia Prime Minister Nouri Maliki is monopolising power
- ISIS led by Abu Bakr al-Baghdadi, obscure figure regarded as a battlefield commander and tactician