IRBIL, Iraq — Dozens of Islamic State militants wearing suicide vests penetrated Iraqi police lines in Mosul on Wednesday, police officers said, retaking ground in a large-scale counterattack and sending residents fleeing.

Starting at around 3 a.m. the militants launched seven car bombs at the front lines south of the Old City, their last remaining foothold, said Lt. Col. Hussein al-Lami, a federal police commander. Simultaneously 25 fighters wearing suicide vests attacked them from behind their lines.

The militants had snuck down the Tigris River and attacked with the assistance of "sleeper cells" which provided vehicles for them, he said.

"They attacked our forces from behind while they were fighting against the car bombs," Lami said, adding that the militants were lashing out in their "dying breath".

After eight months of battle, Islamic State fighters have been penned into the narrow streets and alleyways of Mosul's historic city center as well as a small area around a hospital just to the north.

Iraqi and U.S. military officials estimate that up to 1,000 fighters may remain in the area of just over a square mile. Penned in, they have little choice but to fight to the death.

Lami, who was on leave but receiving updates from his men at the front, said that the militants took over areas of the Dawasa and Dendan neighborhoods, setting light to houses in order to protect themselves from airstrikes by the U.S.-led coalition. However, police forces had managed to retake most of those areas by mid-morning, while suffering casualties, he said, without giving a figure.

Brig. Gen. Faris Radhi, director of the operation room at the federal police headquarters south of Mosul, said that the attack involved 50 suicide bombers. "Clashes are ongoing," he said. Iraqi forces and the U.S.-led coalition are "hunting them one-by-one," he said. "The enemy has used the last card in this attack, which means this is the best they can do."

A resident of Dendan who declined to be named for security reasons as Islamic State had just overrun his neighborhood said federal police had retreated from the main front line after the attack, allowing the militants to take control of the government compound. He had fled the area.

In an official statement Lt. Gen. Raed Shaker Jawdat played down the incident. He described it as a "tactical operation" by police forces aimed at drawing militants out of the Old City and into the secured area of Dendan - where families have returned to their homes - in order to target them with shelling and snipers.