BAGHDAD (AFP) - Al-Qaeda front group the Islamic State of Iraq and the Levant on Tuesday claimed a brazen assault on major prisons in Iraq that freed hundreds of prisoners including senior militant leaders.

"The mujahideen (holy warriors), after months of preparation and planning, targeted two of the largest prisons of the Safavid government," the group said in a statement posted on a jihadist forum, using a pejorative term for Shi'ites.

The statement claimed that "hundreds" of inmates, among them 500 militants, were freed in the attacks on the prisons in Abu Ghraib, west of Baghdad, and Taji, north of the Iraqi capital.

It also said that the operation was the final one in a campaign aimed at freeing prisoners and targeting justice system officials, which was called for in an audio statement attributed to the group's leader Abu Bakr al-Baghdadi last year.

A senior Iraqi security official said initial information indicated that the incident began on Sunday night when prisoners rose up and seized weapons and then contacted militants waiting outside the prisons.

The waiting militants then attacked with mortar rounds, bombs and gunfire, sparking clashes that raged for 10 hours. At least 20 security forces members and 21 inmates died in the unrest.

At least 500 prisoners managed to escape, according to Iraqi MPs.