The rapid advance south by the Islamic State in Iraq and the Levant (ISIL) towards Baghdad appeared to slow over the weekend, but fierce fighting erupted in the town of Tal Afar 40 miles west of Mosul near the Syrian border.
ISIL fighters and other Sunni Muslim armed groups have stormed several towns on the road to Baghdad after seizing Mosul nearly a week ago - an offensive which only stalled as it approached the mainly Shi'ite capital.
The advance alarmed both Prime Minister Nuri al Maliki's Shi'ite supporters in Iran, and officials in the United States, which helped bring him to power after the 2003 invasion that toppled Sunni dictator Saddam Hussein.
US President Barack Obama said on Friday he was reviewing military options, short of sending troops, to combat the insurgency, and Iran held out the prospect of working with its arch-enemy the US to help restore security in Iraq.
Maliki's security forces and allied militias regained some territory on Saturday, easing the pressure on his Shi'ite-led government, and officials said they were regaining the initiative. Maliki has vowed to rout the insurgents.
But yesterday's fighting in Tal Afar, a majority Turkomen town which is home to both Shi'ites and Sunnis, showed how volatile the deepening sectarian split has become.
Residents in Sunni districts accused Shi'ite police and army forces of launching mortar fire at their neighbourhoods, prompting ISIL forces stationed outside the town to move in.
"The situation is disastrous in Tal Afar. There is crazy fighting and most families are trapped inside houses, they can't leave town," a local official said. "If the fighting continues, a mass killing among civilians could result."
In Baghdad yesterday a suicide attacker detonated explosives in a vest he was wearing, killing at least nine people and wounding 20 in a crowded street in the centre of the capital.
At least six people were killed, including three soldiers and three volunteers, when four mortars landed at a recruiting centre in Khlais, 30 miles north of Baghdad.
Volunteers were being gathered by the Iraqi army to join fighting to regain control of the northern town of Udhaim from ISIL militants.
They were some of the thousands who responded to a call by the country's most influential Shi'ite cleric to take up arms and defend the country against the hardline insurgents, many of whom consider the Shi'ites to be heretics.
Pictures distributed on a Twitter account said to be set up by ISIL fighters from Salahuddin province appeared to show dozens of men lying on the ground and being shot by groups of gunmen.
"This is the fate of the Shi'ites which Nuri brought to fight the Sunnis," a caption on one of the pictures reads.
Across the border, the Syrian government launched an air raid near ISIL's headquarters in the eastern city of Raqqa.
Raqqa, the first and only Syrian city to fall to insurgents since Syria's conflict began more than three years ago, has been a major base for ISIL since it evicted rival rebels including al Qaeda's Syrian affiliate during in-fighting this year.