A wave of attacks in Baghdad and north of the capital killed 91 people in Iraq's deadliest day in more than two years after Al-Qaeda warned it would mount new attacks and sought to retake territory.

Officials said at least 172 people were wounded as a result of 21 different attacks mounted in 13 cities, shattering a relative calm which held in the lead-up to the start on Saturday of the holy Muslim fasting month of Ramadan.

In Monday's deadliest attack, a string of roadside bombs and a car bomb, followed by a suicide attack targeting emergency responders, in the town of Taji killed at least 42 people and wounded 40 others, according to two medical officials.

"I heard explosions in the distance so I left my house and I saw a car outside," said 40-year-old Taji resident Abu Mohammed, who added that police inspectors concluded the vehicle was a car bomb.

"We asked the neighbours to leave their houses, but when they were leaving, the bomb went off."

Abu Mohammed said he saw an elderly woman carrying a newborn baby die, as well as the policeman who first concluded the car was packed with explosives.

An AFP reporter at the scene said a row of houses were completely destroyed, and residents were rummaging through the rubble in search of victims and their belongings.

In Baghdad, meanwhile, a car bomb outside a government office responsible for producing identity papers in the Shiite bastion of Sadr City killed at least 12 people and wounded 22 others, security and medical officials said.

"This attack is a terrible crime against humanity, because they did it during Ramadan, while people are fasting," said one elderly witness who declined to be identified.

An AFP journalist said eight nearby cars were badly burned and many of the victims of the 9:30 am (0630 GMT) attack could not be identified because their papers were inside the offices that were targeted.

Two other explosions in the Baghdad neighbourhoods of Husseiniyah and Yarmuk killed at least three people and left 21 others wounded, while a car bomb in the town of Tarmiyah, just north of Baghdad, hurt nine people, officials said.

Checkpoint shootings and bomb blasts in restive ethnically-mixed Diyala province killed 11 people and left 40 others wounded, security officials and Doctor Ahmed Ibrahim from the main hospital in provincial capital Baquba said.

Insurgents also launched attacks on a military base near the town of Dhuluiyah, killing at least 15 Iraqi soldiers and leaving two others wounded, according to two security officials.

Nine bomb blasts, some of them minutes apart, meanwhile killed seven people and wounded 29 in Kirkuk city and the eponymous province's towns of Dibis and Tuz Khurmatu.

Another car bomb, near a Shiite mosque in the town of Dujail, killed a woman and wounded four others.

The attacks came a day after a spate of bombings across Iraq killed at least 17 people and wounded nearly 100 others. Monday's toll was the highest since May 10, 2010, when 110 people were killed.

The latest violence comes after the country suffered a spike in unrest in June when at least 282 people were killed, according to a tally, although government figures said 131 Iraqis died.

Although those figures are markedly lower than during the peak of Iraq's communal bloodshed from 2006 to 2008, attacks remain common.

There was no immediate claim of responsibility for Monday's attacks, but Al-Qaeda's front group in Iraq has warned in recent days that it seeks to retake territory in the country.

The Islamic State of Iraq warned in an audio message posted on various jihadist forums that it would begin targeting judges and prosecutors, and appealed for the help of Sunni tribes in its quest to recapture territory it once held.

"We are starting a new stage," said the voice on the message, purportedly that of Abu Bakr al-Baghdadi, who has been leader of the Islamic State of Iraq 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 Baghdadi.

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."

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