Militants seized the Iraqi city of Tikrit yesterday but their assault on Samarra was repulsed as a lightning jihadist offensive launched in second city Mosul swept closer to Baghdad.
Since the Islamic State of Iraq and the Levant began their spectacular assault in Mosul late Monday, militants have captured a large swathe of northern and north-central Iraq, prompting as many as half a million people to flee their homes.
The speed with which ISIL and its allies have advanced after their seizure Tuesday of Mosul -- a city of two million people -- has sent alarm bells ringing not only in Baghdad but in Western capitals.
The United States yesterday said it "stands ready" to help Iraq to confront the jihadists. Washington called the ISIL as threat to the region.
Washington was committed to "working with the Iraqi government and leaders across Iraq to support a unified approach against ISIL's continued aggression," State Department spokeswoman Jen Psaki told reporters, adding that additional US assistance was in the pipeline.
Iraq's arch rival Iran also offered support to Baghdad against "terrorism".
The Syrian government, which if fighting it own battle agaist ISIL, said it was ready to help Baghdad.
It has also triggered a hostage crisis for Ankara, which is scrambling to secure the release of 48 Turks taken hostage by the jihadists. Militants stormed the Turkish consulate in Mosul and kidnapped the people including the head of the mission, a Turkish official said.
In a statement on Twitter, ISIL vowed that it would "not stop this series of blessed invasions" that has seen the fall of the whole of Nineveh province in the north and swathes of Kirkuk and Saleheddin provinces further south.
Tikrit -- hometown of executed dictator Saddam Hussein -- was the second provincial capital to fall in as many days as the jihadists and their allies captured a string of mainly Sunni Arab towns where resentment against the Shia-led government runs deep.
After Tikrit's fall, the operation spread down the main highway towards Baghdad, with militants battling security forces on the northern outskirts of Samarra, just 110 kilometres from the capital.
Iraqiya state television said security forces launched air strikes on them, and witnesses said the clashes ended without the militants entering the city.
Although Samarra too is mainly Sunni Arab, it is super sensitive for the government as it is home to a shrine revered by the country's Shia majority. A 2006 bombing of the mausoleum by al-Qaeda sparked a Shiite-Sunni sectarian conflict that left tens of thousands dead.
The lightning advance poses significant challenges to Baghdad, with the New York-based Eurasia Group risk consultancy saying they would be bolstered by cash from Mosul's banks, hardware from military bases and hundreds of men they freed from prison.
Prime Minister Nuri al-Maliki has responded by asking parliament to declare emergency rule and saying citizens would be armed to fight them.
Known for its ruthless tactics and suicide bombers, ISIL is arguably the most capable force fighting President Bashar al-Assad inside Syria as well as the most powerful militant group in Iraq.
ISIL is led by the shadowy Abu Bakr al-Baghdadi and backed by thousands of local and foreign fighters.
It appears to be surpassing al-Qaeda as the world's most dangerous jihadist group.