Iraqi forces and Shia units are involved in heavy fighting to retake the country's largest oil refinery from the Islamic State of Iraq and the Levant (ISIL) group.

ISIL holds large sections of the Beiji refinery complex in northern Iraq where police, soldiers and elite special forces are holding out.

Speaking from Beiji, a member of the elite forces said they were aiming to retake the whole area.

"We stormed a building inside the Beiji refinery with the support of [Shia unit] Hashid Shaabi. As you can see, we are inside the refinery now. God willing, within the coming few hours, we will liberate the whole refinery and the area outside the refinery with the help of Almighty God," he said.

According to Hashid Shaabi, an area of 50km south of the town of Beiji, including the villages of al-Hajjaj and the rural area of al-Mazraa, has been retaken.

Control of the refinery has been hotly contested for months.

Smoke from fire

Iraqi government forces recaptured the Beiji refinery from ISIL in November 2014, lost control of it again and then recaptured it in April.

ISIL broke back into the perimeter of the complex earlier this month and have been slowly gaining ground.

Its fighters set large parts of the facility on fire on Monday in efforts to thwart advances by Iraqi security forces.

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Smoke could be seen as the Iraqi army and Shia units  advanced to within 2km of the facility, which lies about 200km from the capital Baghdad.

Colonel Maan al-Saeedi, the commander of the second federal police brigade, told Al Jazeera: "ISIL has rigged [Beiji] with booby-trapped trenches, sand barracks and roadside bombs.

"We are hoping that our forces will overcome these obstacles, the enemy is desperate and lost manpower and firepower and therefore is trying different methods to halt our advance."

A Pentagon spokesperson said this month that the refinery was not operational and its primary importance at the moment was that it sat astride a route to Mosul.

Mosul is the Iraqi city where an ISIL leader last year proclaimed a caliphate to rule over all Muslims.

Bodies exhumed

In other Iraq-related developments, authorities have exhumed the remains of 470 people believed to have been executed by ISIL near the city of Tikrit last year, according to Iraq's health minister.

"We have exhumed the bodies of 470 Speicher martyrs from burial sites in Tikrit," Adila Hammoud announced on Thursday in Baghdad, referring to the nearby military base that the massacre was named after.

In June 2014, armed men belonging or allied to ISIL abducted hundreds of young, mostly Shia recruits from Speicher base, just outside Tikrit.

They were then lined up in several locations and executed one by one, as shown in pictures and footage that has emerged since.

The highest estimate for the number of people killed in one of the worst atrocities ever committed by ISIL stands at 1,700.

Some of their bodies were pushed into the Tigris River and others hastily buried in locations that were discovered when government and allied forces retook Tikrit from the fighters about two months ago.

Source: Al Jazeera and agencies