BAGHDAD -- A string of car bombs tore through busy shopping streets in several Baghdad neighbourhoods on Wednesday night killing 51 people as the army announced that one of its airstrikes had killed 60 militants in the northern city of Mosul.

Baghdad police said the first attack was a pair of car bombs that exploded in the densely populated Shiite neighbourhood of Sadr City, killing 31 people, followed by another bomb in the nearby area of Ur that claimed another 11 lives.

Nine more people were killed in the southeast of the city shortly afterward by two more car bombs.

Baghdad has been on edge since Sunni militants led by the radical Islamic State group conquered large swaths of the country's north, including the second largest city of Mosul. While the fighters have stopped short of advancing on the capital there has been a steady campaign of car bombs in the city, though none this deadly.

The attack came as state-run television announced a rare government victory with an airstrike against a key building in Mosul that killed some 60 suspected militants earlier in the day.

The report, which cited unnamed intelligence officials, could not be independently verified, nor did it say whether any civilians had been killed in the strike on the northern city of Mosul.

The report said the strike freed about 300 people held by the Islamic State group at a downtown Mosul prison it had been using as a religious court and detention centre, without elaborating.

A Mosul resident, speaking on condition of anonymity fearing for his own safety, told The Associated Press over the phone that families of the prisoners rushed to the site to help their relatives after the airstrike.

"The prison was partly damaged in the airstrike," he said. He said he did not know if there were casualties.

Phone calls to Iraqi officials rang unanswered Wednesday.

The onslaught by the Islamic State backed by local Sunni militants has plunged Iraq into its worst crisis since the withdrawal of U.S. troops in 2011.

The group since has imposed a self-styled caliphate in territory it controls in Iraq and Syria, imposing their own harsh interpretation of Islamic law.

Iraqi government forces and allied Sunni tribal militiamen have been struggling to dislodge the militants with little apparent progress.

A few hours after the reported airstrike, Islamic State group militants broke into a nearby hospital, ordered the morgue employees to stay in a separate room and put a number of corpses inside a refrigerator, a medical official said on condition of anonymity for his own safety.

Also Wednesday, police discovered eight bullet-riddled and handcuffed corpses around Baghdad.

Six of them were found in the town of Taji, about 20 kilometres (12 miles) north of Baghdad, a police officer said. All were men between 25 to 35 years old.

Two other dead bodies, a man and woman, were found in the southeastern district of Zafaraniyah, another police officer said.

The number of corpses found with gunshot wounds has been on rise recently in a grim reminder of the sectarian killing that engulfed Iraq in 2006 and 2007. Then, both Shiite and Sunni death squads roamed the streets and raided homes to round up people. Their corpses would later be found by police, often mutilated.