An Al-Qaeda front group on Tuesday claimed brazen assaults on Iraqi prisons that freed hundreds of militants including top leaders, killed over 40 people and threaten to further erode confidence in the government.

The attacks on the prisons in Abu Ghraib, west of Baghdad, and Taji, north of the capital, illustrate the growing reach of militants in Iraq and the deteriorating security situation in the country, where more than 600 people have been killed in violence so far this month.

"The mujahideen (holy warriors), after months of preparation and planning, targeted two of the largest prisons of the Safavid government," said a statement signed by the Islamic State of Iraq and the Levant, using a pejorative term for Shiites.

The statement claimed that "hundreds" of inmates, among them 500 militants, were freed in the attacks.

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.

The statement, posted on a jihadist forum, came as security forces were hunting intensely for the escapees, said by MPs to number at least 500 before they are able to rejoin the ranks of the militants.

"Dark days are waiting for Iraq. Some of those who escaped are senior leaders of Al-Qaeda, and the operation was executed for this group of leaders," a high-ranking security official told AFP on condition of anonymity.

"Those who escaped will work on committing acts of revenge, most of which might be suicide attacks," the official said.

In what appears to have been a carefully-planned operation, militants waiting outside the prisons launched their attacks after inmates inside began rioting.

"The first information we have indicates that the incident started from inside the prison," the security official said.

"There were riots and then the prisoners took control of some guns and called the groups that were waiting outside."

Militants then attacked with mortar rounds, bombs and gunfire, sparking clashes with security forces that raged for 10 hours.

At least 20 security forces members and 21 inmates died in the unrest.

Officials have declared "a curfew around the two prisons, where ongoing search operations are being conducted," justice ministry spokesman Wissam al-Fraiji told AFP.

Fraiji said that 108 escaped prisoners had been recaptured, and reinforcements from the interior and justice ministries have been sent to the two prisons.

MP Hakem al-Zamili, a member of parliament's security and defence committee, confirmed that senior Al-Qaeda members had escaped, and expressed fear that they would return to haunt Iraq again.

"Most of the inmates who escaped from Abu Ghraib prison were senior members in the Al-Qaeda organisation and (had been) sentenced to death," Zamili told AFP.

"These terrorists will go to Syria to return to the (Al-Qaeda) organisation and return again to Iraq to carry out terrorist attacks against the Iraqi people," he said.

Al-Qaeda-linked fighters are among those battling the regime of President Bashar al-Assad in Syria, which shares a long border with Iraq.

The conflict pits mainly Sunni Muslim rebels against Assad, a member of the Alawite sect, which is an offshoot of Shiite Islam.

It has spilled over the border on several occasions and raised tensions in Iraq. Both Iraqi Sunnis and Shiites have travelled to Syria to fight.

The prison assaults and escapes illustrate the woeful security situation in Iraq and threaten to undermine confidence in the government, experts told AFP.

"The escape of prisoners in this organised way from the biggest prisons in Iraq is another sign of the deterioration of security in Iraq in general, and Baghdad in particular," said Hamid Fadhel, a policial science professor at Baghdad University.

"It seems that the security situation is the victim of the political conflict in Iraq today," he said, referring to long-running disputes among the country's various political factions that have paralysed the government, with almost no major legislation passed in years.

The assaults and mass escapes "affect people's trust in the security forces and in the government, because people will start to worry that the criminal can commit a crime, go to prison, and then get out easily," said Ali al-Haidari, an Iraqi expert in security and strategic affairs.

"What happened puts the government in a very embarrassing situation. What we saw was a huge attack with large numbers of fighters, and it seems that the guards of the two prisons were not able to stop such an attack," he said.