Armed fighters believed to be part of the Islamic State in Iraq and the Levant have seized the northern Iraqi province of Nineveh and freed hundreds of prisoners, government officials say.

Overnight, hundreds of fighters launched an assault on the provincial capital Mosul, 350km north of Baghdad, engaging in combat with troops and police, the officials said on Tuesday.

"The city of Mosul is outside the control of the state and at the mercy of the militants," an Interior Ministry official told the AFP news agency, making it the second city to fall to anti-government forces this year.

Turkish media also reported on Tuesday that 28 Turkish lorry drivers were taken hostage by ISIL fighters in Mosul, Iraq's second largest city.

In recent days, fighters have launched major operations in Nineveh and four other provinces, killing scores of people and highlighting both their long reach and the weakness of Iraq's security forces.

Al Jazeera correspondent Imran Khan, reporting from Baghdad, said: "the intelligence estimates Iraq has released suggests it's not just Iraqi fighters. There are foreign fighters who have come to fight for ISIL."

Mosul is the second city to be captured by rebels this year, after the central government lost control of Fallujah.

Control of prisons

Before Mosul fell on Tuesday, ISIL fighters took control of the governor's headquarters, prisons and television stations, reports said.

The Associated Press news agency reported that the group freed about 1,400 prisoners held in the city's jails.

A pro-ISIL Twitter feed boasted that fighters had released about 3,000 prisoners from three facilities.

Describing the assault, Ali Mahmoud, media official for Nineveh, said rebels armed with machineguns and rocket-propelled grenade launchers stormed the provincial headquarters building in Mosul late on Monday night.

He said the attackers were able to overpower the building guards after a short firefight.

He confirmed accounts by Mosul residents that many of the police and army forces that had been stationed in the city had disappeared by Tuesday.

Atheel al-Nujaifi, Nineveh's governor, was in a nearby guest house but managed to escape from the area unharmed, Mahmoud said.

Osama al-Nujaifi, the parliament speaker and brother of Atheel al-Nujaifi, said he had asked the US ambassador in Baghdad for help in order to stop what he described as "a foreign invasion by ISIL".

Nujaifi said he had also requested the help of the Kurdish peshmerga but received no response from the semi-autonomous Kurdistan Regional Government, which controls the fighters.