Baghdad could be the next city to fall to militants in Iraq, with the chief of the extremists who have swept the north of the country vowing to bring "rage" to the capital.
Across the country, the Iraqi army has abandoned its position. Iraqi Kurdish forces say they are now in full control of the northern oil city of Kirkuk, the only bulwark remaining against the insurgent sweep. "The whole of Kirkuk has fallen into the hands of peshmerga [Kurdish fighters]," Kurdish spokesman Jabbar Yawar told Reuters. "No Iraq army remains in Kirkuk now."
The Iraqi government is believed to have privately asked the US for help including possible airstrikes and drone attacks to drive back the militia.
In a statement translated by US intelligence officials on the monitoring website SITE, Abu Mohammed al Adnani, the head of the Islamic State of Iraq and the Levant (ISIS) said he would soon take Baghdad.
"The battle is not yet raging, but it will in Baghdad and Karbala," he said. "Do not relent against your enemy. Put on your belts and get ready."
ISIS, a radical splinter group whose tactics were once deemed too extreme even for al Qaeda, seized the northern city of Tikrit on Wednesday, the birthplace of Sadaam Hussein. They have driven 500,000 out of Mosul, Iraq's second city, after the country's army abandoned their positions and fled. Militants have also taken 48 hostages from the Turkish consulate in the city.
The insurgents in Tikrit set fire to government buildings but have now headed south, according to Sky News, who said battalions of militia men were now exchanging shots with security forces on northern outskirts of Samarra, just 70 miles from the capital.
Thus far, Iraqi security forces had been able to halt the advance of jihadist fighters towards Baghdad.
The BBC said its reporters have seen signs the militants" may be preparing for an offensive from the west, where they control the city of Fallujah, 43 miles from Baghdad".
An American defence official, speaking to AFP on condition of anonymity, said the Iraqi government had privately asked for help and the US is considering sending "more weaponry".
Iraqi Prime Minister Nouri al Maliki has called an emergency session of parliament on Thursday to vote to give the premier more power to tackle the militants.
The UN Security Council said the humanitarian situation around Mosul is "dire and is worsening by the moment".
Secretary General Ban Ki-moon called on "the international community to unite in showing solidarity with Iraq as it confronts this serious security challenge".
In Britain, Iraqi-born Conservative MP Nadhim Zahawi blamed the West's failure to intervene in Syria, which he said had help fuel the takeover of large parts of Iraq by Islamist extremists.
Iraq-born Nadhim Zahawi blamed US policy in the wake of the 2003 invasion to oust Saddam Hussein for sowing the seeds of the insurgency.
But he said the "divisive sectarian" rule of Iraqi prime minister Nouri al-Maliki and the lengthy and increasingly extremist-led civil war in Syria were fanning the flames.
Zahawi told BBC Radio 4's Today that the US might consider providing air support to Iraqi troops in efforts to retake territory.
"This really goes back to the US policy in Iraq when Paul Bremer was administering the country. He took a decision, backed by the American government, to disband the Iraqi Army - the 'de-Ba'athification' process," he said.
"He sent home 700,000 men with weapons, no jobs, no job prospects, predominantly Sunni, to their families.
"Some of those people organised and this chap Abu Bakr al-Baghdadi, who is the emir of Isis, is one of those men.
"It is compounded by Maliki's divisive, sectarian policies. Maliki is the commander in chief and the minister of defence - because he hasn't appointed anyone - so he decides who gets promoted within the army."
"The Americans have sold Maliki some fighter jets. I don't know whether they would go further and heed his call for further intervention with some air cover for the Iraqi military to be able to retake some of these cities," Zahawi said.
The MP - who was among those who supported strikes on the Syrian regime of Bashar Assad last summer which were effectively voted down by the Commons - said failure to act there had opened the door to groups such as Isis.
"I argued for very limited intervention to take the chemical weapons out of the game in Syria to protect the silent majority of innocent people," he said.
"Early intervention in Syria, supporting people who were working towards replacing Bashar al Assad with a moderate democracy which is inclusive would have been the right move because otherwise the message you send to the silent majority is actually you don't care about them and that is where these groups thrive because they go in and they are brutal and they take over and fill that vacuum."