Al-Qaeda's branch in Iraq has claimed responsibility for last week's jail break in Tikrit as well as a series of attacks around the country that left at least 38 people dead.
In a statement posted on a website, the Islamic State of Iraq said on Friday that the attacks, mostly bombings using explosive-rigged parked cars, were launched against "carefully selected targets" in Baghdad and some other provinces.
The announcement came as police told the Associated Press news agency that a twin bombing in southeast Baghdad had killed five people, including four Shia worshippers.
The blast hit the al-Sadrein mosque and a nearby police checkpoint as Friday prayers ended in the mostly Shia neighbourhood of Zafaraniyah.
They said at least 26 worshippers were wounded.
A medical official in a nearby hospital confirmed the casualty figures.
All officials spoke on condition of anonymity because they are not authorised to talk to the media.
The Islamic State of Iraq said in its statement that both the freeing of its prisoners and the assassination of officials are top priorities, part of a general offensive intended to retake Sunni towns and districts that were once al-Qaeda strongholds.
Last Sunday's co-ordinated blasts, stretching from the northern city of Kirkuk through Baghdad to the south, mostly targeted Shia neighbourhoods and Iraqi security forces. In all, at least 26 people died.
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The group said these attacks were in response to "atrocities committed by the Shia government against the Sunni prisoners".
The Islamic State of Iraq also claimed responsibility for the September 27 prison break in Tikrti, Saddam Hussein's home town, in which prisoners seized weapons and clashed with security forces in an hours-long standoff.
Twelve people died and dozens of al-Qaeda inmates escaped.
"The tight security measures did not prevent the fighters of the Islamic State from carrying out this operation with the maximum accuracy," the statement said.
September was the deadliest month in Iraq in more than two years, with 365 people killed in violence that included waves of nationwide attacks, official figures show.
Fighters are regarded as weaker than when violence reached its peak in 2006 and 2007, but they remain capable of carrying out mass-casualty attacks across the country.
The statistics compiled by the health, interior and defence ministries showed that 182 civilians, 88 police and 95 soldiers were killed in attacks in September.
Another 683 people were wounded, which includes 453 civilians, 110 police and 120 soldiers, according to the figures.
It was the highest monthly toll given by the government since August 2010, when figures showed 426 people killed and 838 wounded in attacks.