RAMADI, Iraq (AFP) - Jihadists took students and staff hostage at Anbar University in the Iraqi city of Ramadi Saturday, with one of the prisoners saying they had been threatened.

That prompted an assault by security forces to free them.

Gunmen from the Islamic State of Iraq and the Levant infiltrated the university from the nearby Al-Tasha area, killed its guards and then blew up a bridge leading to its main gate, police said.

An AFP journalist said special forces spearheaded an assault to retake the campus, and reported hearing heavy gunfire.

Around 1,000 students managed to escape earlier, while more fled during the military operation, which also included soldiers and police, the journalist said.

It was not immediately clear how many students are registered at the university, or how many of them are still being held.

Before the military assault began, a student telephoned from inside the university to say she and other women were ordered to gather in one place, after which the leader of the militants addressed them.

"We will teach you a lesson you will never forget," she told AFP he had said to them.

In December, a crisis broke out in the desert province of Anbar, west of Baghdad, when security forces dismantled a longstanding Sunni Arab protest camp near provincial capital Ramadi.

Anti-government fighters subsequently seized control of parts of Ramadi and all of Fallujah, to its east, and security forces have so far failed to drive them out.

The United Nations said on Friday that the conflict in Anbar is believed to have forced nearly 480,000 people from their homes.

They join some 1.1 million others displaced by past years of violence in Iraq.

On Friday, heavy fighting between militants and security forces and twin suicide bombings targeting a minority group, killed at least 36 people in the northern province of Nineveh.

A day earlier, militants seized several parts of Samarra in a major assault that was only repelled after house-to-house fighting and helicopter strikes.

Violence is running at its highest levels since 2006-2007, when tens of thousands were killed in sectarian conflict between Iraq's Shiite majority and Sunni Arab minority.

More than 900 people were killed last month, according to figures separately compiled by the United Nations and the government.

So far this year, more than 4,300 people have been killed, according to AFP figures.

Officials blame external factors for the rise in bloodshed, particularly the civil war in neighbouring Syria.

But analysts say widespread Sunni Arab anger with the Shiite-led government has also been a major factor.