Iraq gunmen vacate uni after raid

Jihadist gunmen have withdrawn after taking staff and students hostage at a university campus in the Iraqi city of Ramadi.

Jihadists have taken staff and students hostage at a university campus in the Iraq city of Ramadi. (AAP)

Militants stormed a university in Iraq's restive Anbar province, briefly taking dozens of students hostage before withdrawing from the school amid gunfire, officials and witnesses say.

On Saturday morning the gunmen killed three police officers on guard at Anbar University's gate, a police and a military official said.

The gunmen then detained dozens of students inside a university dorm, the officials said.

Ahmed al-Mehamdi, a student who was taken hostage, said he awoke to the crackle of gunfire, looked out the window and saw armed men dressed in black racing across the campus.

Minutes later, the gunmen entered the dormitory and ordered everybody to stay in their rooms while taking others away, he said.

The Shi'ite students at the school were terrified, al-Mehamdi said, as the gunmen identified themselves as belonging to an al-Qaeda splinter group known as the Islamic State of Iraq and the Levant.

The Sunni terror group, fighting in Syria with other rebels trying to topple President Bashar Assad, is known for massive, bloody attacks in Iraq as well often targeting Shi'ites that they view as heretics.

The Islamic State did not immediately claim the attack on the school, which says it has more than 10,000 students, making it one of the country's largest.

Several hours later, gunmen left the university under unclear circumstances. Students then boarded buses provided by the local government to flee the school, though gunfire erupted as security forces attacked retreating militants, police said.

"We thank God that this crisis ended almost peacefully and no student was hurt as far as I know," al-Mehamdi said by mobile phone from inside a bus that took him to safety.

Security officials said authorities wanted to wait for bomb disposal experts before entering any building on campus out of fears that the fleeing gunmen planted explosives.

Iraq is currently grappling with its worst surge in violence since the sectarian bloodletting of 2006 and 2007, when the country was pushed to the brink of civil war despite the presence of tens of thousands of US troops.

The latest violence has been fueled by Sunni Muslim anger at the Shi'ite-led government in Baghdad, as well as the civil war in neighbouring Syria.