Baghdad: Al-Qaeda's front group in Iraq said it would begin targeting judges and prosecutors, and try to help its prisoners break out of Iraqi jails, in an Internet audio message purportedly left by its leader.

The message posted on various jihadist forums said the Islamic State of Iraq (ISI) would look to retake territory, and appealed for Sunni tribes to provide support and send fighters.

"We are starting a new stage," the voice on the audio message, purportedly that of Abu Bakr al-Baghdadi who has been leader of ISI since May 2010.

"The first priority in this is releasing Muslim prisoners everywhere, and chasing and eliminating judges and investigators and their guards."

It was not possible to verify whether the voice was that of the ISI leader.

Baghdadi added: "On the occasion of the beginning of the return of the state to the areas that we left, I urge you to carry out more efforts, and send your sons with the mujahedeen to defend your religion and obey God."

The ISI leader rose to his position after his predecessor Abu Omar al-Baghdadi was killed in a joint US-Iraqi raid on a safehouse in April 2010.

Al-Qaeda in Iraq is regarded by Iraqi officials to be significantly weaker than at the peak of its strength in 2006 and 2007, but it is still capable of spectacular mass-casualty attacks across the country.

Earlier this month, a truck bomb blamed on Al-Qaeda killed 25 people at a crowded market south of Baghdad.