Iraqi military forces have reached the centre of the northern oil city of Beiji to try to break a siege by the Islamic State in Iraq and the Levant (ISIL) on the country's biggest refinery, a colonel and a witness have told the Reuters news agency.

The armed ISIL group captured Beiji city in June during a lightning advance through northern Iraq.

Since then, they have surrounded the oil refinery and halted its production while a detachment of government troops has held out for months under siege inside it.

A military colonel told the Reuters news agency on Sunday that Iraqi troops entered the a city of about 200,000 people from the south and west, and took over the al-Tamim neighbourhood and city centre.

ISIL had placed bombs along roads in Beiji and deployed snipers to keep government forces from advancing, tactics used in other cities held by the group.

"There are IEDs [improvised explosive devices] and snipers that are slowing down the advance, but the presence of the air force has facilitated the process of dismantling the IEDs in order to push forward," said the colonel.

"The areas taken so far are 6km away from Beiji's refinery," he added. He said 12 fighters had been killed.

'Violent confrontations'

Beiji resident Sultan al-Janabi told Reuters by telephone from his house that clashes had been raging since the advance, the first time security forces had reached the city centre since launching a new encirclement strategy at the end of last month.

"Violent confrontations are taking place in Beiji right now. I've been hearing continuous fire and loud bangs," said Janabi.

ISIL has captured large swathes of land in Iraq and Syria, declaring what it calls a cross-border Islamic "caliphate" and committing several atrocities in the process.

The US-led coalition's fighter jets have been bombing the group's targets in Iraq and Syria, supporting the ground struggle carried out by the central government and Kurdish forces.

On Saturday, US-led air strikes targeted a gathering of ISIL leaders in Iraq near the al-Qaim border crossing between Iraq and Syria.

ISIL considers Shia Muslims, Christians and members of other religions to be heretics and frequently attacks them.

Source: Agencies