Militants have seized the Iraqi city of Tikrit, but their assault on Samarra has been repulsed.
The lightning jihadist offensive launched in the second city Mosul is also sweeping closer to Baghdad.
Since the Islamic State of Iraq and the Levant (ISIL) began their spectacular assault in Mosul late on 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 of Mosul – a city of two million people – has sent alarm bells ringing not only in Baghdad but in Western capitals.
It has also triggered a hostage crisis for Ankara, which is scrambling to secure the release of 48 Turks taken hostage by the jihadists.
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, home 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 Shiite-led government runs deep.
“All of Tikrit is in the hands of the militants,” a police colonel said of the Salaheddin provincial capital, which lies roughly half way between Baghdad and Mosul.
A police major said the militants had freed some 300 inmates from a prison there.
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.
It was not immediately clear what became of the attackers.
Militants had already tried to seize the city late last week, and were only halted by a massive deployment of troops, backed by tribal militia and air power.
Prime Minister Nuri al-Maliki has responded by asking parliament to declare emergency rule and saying citizens would be armed to fight them, while Washington has warned that ISIL threatened the entire region.