Islamic State crisis: Thousands flee Iraqi advance on Tikrit Published duration 5 March 2015

image copyright Reuters image caption Iraqi government forces have been slowed by roadside bombs and booby traps

A military operation to retake the Iraqi city of Tikrit from Islamic State (IS) has caused about 28,000 people to flee their homes, the UN says.

Those displaced are headed towards the city of Samarra, the UN said, but many families are stranded at checkpoints.

Aid convoys carrying relief supplies are being sent to the area by UN agencies to help those affected.

The operation to retake Tikrit, involving some 30,000 soldiers and Shia militiamen, is now in its fourth day.

They are trying to encircle the IS fighters, but their advance has been slow due to the roadside bombs and booby traps planted since the city was overrun last June, says the BBC's Jim Muir in Beirut.

Iraqi jets and helicopters are supporting the ground troops but US-led coalition aircraft are not involved.

Militants also set fire to oil wells outside the city on Thursday, officials said, creating clouds of smoke in an apparent bid to obscure targets from air strikes.

Iraqi Gen Abdul Wahhab al-Saadi told state TV that the burning oil wells would not affect the operation.

But as the fighting continues, concerns for the civilian population are growing.

A UN statement said: "Military operations in and around Tikrit have precipitated displacement of an estimated 28,000 people to Samarra.

"Field reports indicate that additional displacements are under way and that yet more families remain stuck at checkpoints."

The White House and human rights organisations have also warned against the danger of sectarian reprisals by the Shia militia in the predominantly Sunni area.

Militia leaders have vowed to seek revenge for the massacre of hundreds of soldiers, most of them Shia, at Camp Speicher near Tikrit in June.

On Wednesday, a Iraqi army source told the BBC that government forces had taken control of the village of al-Maibdi, on the road between Tikrit and the Kurdish-controlled city of Kirkuk, as well as the nearby Ajil and Alas oilfields.

image copyright Reuters image caption Shia militias are playing a major role in the offensive alongside Iraqi troops

The road was a key supply route for IS between Salahuddin and Diyala provinces, the source said.

Another official told All Iraq News that the villages of Siha and Mazraat al-Rahim, just to the north of Tikrit in al-Alam district, had also been retaken.

However, the soldiers and militiamen have not breached IS defences around Tikrit and al-Dour, a town 19km (12 miles) to the south, which officials say is another stronghold of the jihadist group.