IRBIL: Iraqi special forces entered the Mosul University on Friday, a tactical achievement and an incremental step in in battling Islamic State militants for control of the city, according to senior Iraqi officers.

The troops entered the university grounds in the morning hours and managed to secure parts of the compound, which is located in eastern half of Mosul, said two officers, speaking on condition of anonymity because they were not authorized to talk to reporters.

Brig. Gen. Haider Fadhil confirmed the account and added that Iraqi forces were now fighting fierce battles with IS fighters inside the complex.

The development comes a day after Iraqi army forces north of the city linked up with troops pushing in from the city's eastern edge.

The sprawling university compound, a symbolic landmark in Iraq's second-largest city, was once used by IS militants as a base. Iraqi officials had said that the militants also used the school's chemistry labs to produce chemical weapons.

Iraqi special forces Lt. Gen. Abdul-Wahab al-Saadi told The Associated Press that it is unclear if the complex was still being used as an IS base.

Under heavy bombardment from US-led coalition airstrikes, IS has repeatedly moved bases under cover of civilians. Earlier this month Iraqi forces retook a hospital that had been converted into an IS base.

Meanwhile, south of Mosul, the United Nations warned of an oil spill, near the town of Qayara that was retaken from IS militants in August.

The fight against IS over the past two years in Iraq has left large swaths of destruction in its wake, destroying key buildings and infrastructure.

The warnings came in a report released this week on environmental damages caused by oil fires intentionally started by retreating IS militants.

U.N. satellite images published on Tuesday show environmental damage from the fires. The world body says 11 of the fires have been extinguished, but 29 continue to burn, sending thick black smoke into the air.

The report warns the spill, described as new, is close to a tributary of the Tigris River, which in turn could mean an oil leak into the key artery, and that parts of the spill are on fire.

As the Mosul operation enters its fourth month Iraqi forces have retaken about a third of the city that has been under tight IS control for more than two years. While Iraqi officials initially pledged the city would be "liberated" this year, the fight is likely last many more months.