Bombings across Iraq killed 17 people on Sunday, the country's deadliest day in nearly three weeks, as Al-Qaeda warned it would target judges and prosecutors, and look to free Muslim prisoners.

The violence, the worst of which struck just before the Iftar meal in which Muslims break the daytime fast observed during the holy month of Ramadan, also wounded 100 people and struck towns across Iraq's north, south and west.

In the deadliest attack, two car bombs minutes apart at the main market in the town of Mahmudiyah killed at least 10 people and wounded 36, two medical officials said.

The first bomb exploded at around 7:15 pm (1615 GMT) while a second struck minutes later in the town, which lies 30 kilometres (18 miles) south of Baghdad in what was a confessionally-mixed area known as the "Triangle of Death" for the high levels of violence during the worst of Iraq's sectarian conflict.

In the nearby town of Madain, just southeast of Baghdad, multiple roadside bombs just before the Iftar meal killed six people and wounded 13, an interior ministry official and a medical source said.

And in the main northern city Mosul, a car bomb near a police headquarters killed a policeman and wounded 15 other people, police Lieutenant Mohammed Khalaf and doctor Mahmud Haddad at the city's hospital said.

Separate car bombings in the western city of Ramadi, capital of predominantly Sunni Arab Anbar province, wounded nine people, including five policemen, security and medical officials said.

In the central Shiite shrine city of Najaf, a car bomb near a restaurant in the old town wounded 23 people, four them seriously, according to provincial health department spokesman Salim Naama.

Najaf is the site of the mausoleum of Imam Ali, son-in-law and cousin of the Prophet Mohammed and a central figure in Shiite Islam, which draws pilgrims from around the world.

The death toll from the nationwide attacks was the highest since July 3, when 39 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, especially in the area south of Baghdad, in Mosul and in Ramadi.

The vioelnce came as Al-Qaeda's front group in Iraq said in an audio message posted on jihadist forums that it would begin targeting judges and prosecutors, and appealed for the help of Sunni tribes in its quest to retake territory.

"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 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 as significantly weaker than at the peak of its strength in 2006 and 2007, but it is still capable of spectacular mass-casualty attacks.

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