Iraqi Special Forces advancing inside Mosul were hit by a suicide car bomber Saturday as Islamic State fighters battle to keep their hold on Iraqi's second-largest city in the face of an all-assault by ground troops and U.S.-led coalition airstrikes.

The IS fighters struck in an early morning in the Qadisiya neighborhood, which the Iraqi Counter-Terrorism Service troops had entered a day earlier.

The militants, defending one of their last strongholds in their self-proclaimed caliphate, which straddles Iraq and Syria, followed the car bombing with a barrage of gunfire, mortar rounds and rocket-propelled grenades, Iraqi officers said, according to the Associated Press.

The Iraqi armed forces do not release casualty figures, but field medics have noted dozens of killed and wounded since the operation to liberate Mosul began Oct. 17.

Military officers tell Reuters that small groups of militants are using a large network of tunnels and narrow streets in densely populated areas to launch an apparently endless sequence of attacks against advancing Iraqis.

Staff Lt. Col. Ali Fadhel tells Agence France Presse the fighting was particular intense as Iraqi troops were advancing in an effort to surround the Al-Bakr neighborhood.

"There were three car bombs coming out from Al-Bakr toward our positions that we spotted with our drone and hit with our tanks," Salem said, according to the news agency.

Fighters from the Islamic State, known as ISIL or ISIS, seized Mosul two years ago. U.S. military officials estimate about 3,000 to 5,000 ISIS fighters remained when other militants retreated toward their final stronghold in Raqqa, Syria.

To the south, meanwhile, Iraq police to the south have come within 3 miles of Mosul's airport.