Hundreds of convicts, including senior members of al-Qaida, broke out of Iraq's Abu Ghraib jail as comrades launched a military-style assault to free them, authorities said on Monday.

The deadly raid on the high-security jail happened as Sunni Muslim militants are regaining momentum in their insurgency against the Shia-led government.

Suicide bombers drove cars packed with explosives to the gates of the prison on the outskirts of Baghdad on Sunday night and blasted their way into the compound, while gunmen attacked guards with mortars and rocket-propelled grenades.

Other militants took up positions near the main road, fighting off security reinforcements sent from Baghdad as militants wearing suicide vests entered the prison on foot to help free the inmates.

Ten policemen and four militants were killed in the ensuing clashes, which continued until Monday morning, when military helicopters arrived, helping to regain control.

By that time, hundreds of inmates had succeeded in fleeing Abu Ghraib, the prison made notorious a decade ago by photographs showing abuse of prisoners by US soldiers.

"The number of escaped inmates has reached 500, most of them were convicted senior members of al-Qaida and had received death sentences," Hakim al-Zamili, a senior member of the security and defence committee in parliament, told Reuters.

"The security forces arrested some of them, but the rest are still free."

One security official told Reuters on condition of anonymity: "It's obviously a terrorist attack carried out by al-Qaida to free convicted terrorists."

A simultaneous attack on another prison, in Taji, around 12 miles north of Baghdad, followed a similar pattern, but guards managed to prevent any inmates escaping. Sixteen soldiers and six militants were killed.

Sunni insurgents, including the al-Qaida-affiliated Islamic State of Iraq, have been regaining strength in recent months and striking on an almost daily basis against Shia Muslims and security forces amongst other targets.

The violence has raised fears of a return to full-blown conflict in a country where Kurds, Shia and Sunni Muslims have yet to find a stable way of sharing power.

Relations between Islam's two main denominations have been put under further strain from the civil war in Syria, which has drawn in Shia and Sunni fighters from Iraq and beyond to fight against each other.

Nearly 600 people have been killed in militant attacks across Iraq so far this month, according to violence monitoring group Iraq Body Count.

Recent attacks have targeted mosques, amateur football matches, shopping areas and cafes where people gather to socialise after breaking their daily fast for the holy Muslim month of Ramadan.