The al-Qaida-inspired group that led the charge in capturing two key Sunni-dominated cities in Iraq this week has vowed to march on to Baghdad.
The group's actions have raised fears about the Shiite-led Government's ability to slow the assault following the insurgents' gains.
Fighters from the militant group known as the Islamic State of Iraq and the Levant (ISIL) took Saddam Hussein's home town of Tikrit, as soldiers and security forces abandoned their posts and yielded ground once controlled by US forces.
That seizure followed the capture of much of Mosul, Iraq's second-largest city, the previous day.
The group and its allies among local tribesmen also hold the city of Fallujah and other pockets of the Sunni-dominated Anbar province to the west of Baghdad.
A spokesman for ISIL said the group had old scores to settle with Prime Minister Nouri al-Maliki's administration in Baghdad. The Iraqi leader, a Shiite, is trying to hold on to power after indecisive elections in April.
Mr al-Maliki has called on parliament to declare a state of emergency that would give him the "necessary powers" to run the country – something legal experts said could include powers to impose curfews, restrict public movements and censor the media.
The ISIL spokesman, Abu Mohammed al-Adnani, threatened that the group's fighters will take the cities of Karbala and Najaf, which hold two of the holiest shrines for Shiite Muslims.
"March toward Baghdad because there we have an account to settle," he urged followers in a recording posted on websites.
Al-Adnani also said that one of ISIL's top military commanders, Adnan Ismail Najm, better known as Abu Abdul-Rahman al-Bilawi al-Anbari, was killed.
ISIL aims to create an Islamic emirate spanning both sides of the Iraq-Syria border.
The White House has said that the United States was "deeply concerned" about ISIL's continued aggression.
Meanwhile, hundreds of young men crowded in front of the main army recruiting centre in Baghdad after authorities urged Iraqis to help battle the insurgents.
And Britain deployed a humanitarian team to assess the needs of civilians fleeing the violence, International Development Secretary Justine Greening said.