Militants who stormed cities in northern and central Iraq now plan to launch a wave of attacks on the capital Baghdad, their leader has reportedly warned.

In a statement translated by intelligence officials, Abu Mohammed al Adnani, who heads the Islamic State of Iraq and the Levant (ISIS), promised battle would "rage" in the city, and in Karbala further south.

It came as officials in Iraq allegedly asked the US to consider air strikes, including drone attacks, to halt the militants' progress.

Battle will rage

"The battle is not yet raging but it will," al Adnani said, according to the US-based monitoring group SITE.

"Do not relent against your enemy," he added. "Put on your belts and get ready."

ISIS, an al Qaeda splinter group, seized control of Tikrit on Wednesday after a devastating show of strength in Mosul, where more than 500,000 people have fled.

Iraqi police said hundreds of prisoners were freed in Tikrit, the home town of former dictator Saddam Hussein, after militants arrived in a convoy of more than 60 vehicles.

The insurgents took over local government buildings and set fire to the court house before heading back out on the road.

They battled security forces on the northern outskirts of Samarra as they closed within 110km of the capital.

Appeal to America

An American defence official, speaking to the AFP news agency on condition of anonymity, said the Iraqi government had privately asked for help in stopping them.

A formal request has not yet been made, although the US is considering sending "more weaponary", the official said.

American troops pulled out of Iraq in 2011 after a bitter conflict that left 4,800 coalition soldiers dead.

State Department spokeswoman Jen Psaki said the US would support a "unified approach against ISIS' continued aggression", adding it had long warned of the dangers posed by the group.

State of emergency

Iraqi Prime Minister Nouri al Maliki asked parliament to declare a state of emergency that would grant him additional powers to tackle the crisis.

Foreign Minister Hoshyar Zebari said Baghdad will co-operate with Kurdish forces to "flush out these foreign fighters" and said Iraq's leaders have to unite to face a "mortal" threat.

ISIS, which wants to create a Sunni state, or caliphate, straddling the border between Iraq and Syria, has made serious gains in Iraq in the last year, seizing control of Falluja and parts of Ramadi.

It was formed after a split with al Qaeda's international leader, Ayman al Zawahiri.