A suicide bomber made his way into a crowded Shia mosque before blowing himself up in the middle of a packed funeral on Wednesday, killing 42 people and leaving corpses scattered across the floor.

The attack, which also left 75 people wounded, is likely to heighten tensions as Iraq grapples with a political crisis and more than a month of protests in Sunni-majority areas that have hardened opposition against Shia prime minister Nuri al-Maliki.

No group claimed responsibility, but Sunni militants often launch attacks in a bid to destabilize the government and push Iraq back towards the sectarian violence that blighted it from 2005 to 2008.

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The militant struck at the Sayid al-Shuhada mosque in Tuz Khurmatu, 175 kilometres north of Baghdad, and targeted the funeral of a relative of a politician who was killed by gunmen a day earlier.

Niyazi Moamer Oghlu, the secretary general of the provincial council of Salaheddin, which surrounds Tuz Khurmatu, put the toll from the attack at 42 dead and 75 wounded.

"Corpses are on the ground of the Husseiniyah (Shia mosque)," said Shallal Abdul, mayor of Tuz Khurmatu. "The suicide bomber managed to enter and blow himself up in the middle of the mourners."

Among the wounded were officials and tribal leaders, including Ali Hashem Oghlu, the deputy chief of the Iraqi Turkman Front and a provincial councillor in Salaheddin.

The funeral had been for Mukhtar's brother-in-law, who was shot dead in Tuz on Tuesday afternoon.

Wednesday's suicide bomb came after a wave of attacks a day earlier killed 26 people and wounded dozens more, shattering a relative calm after a spate of deadly violence last week.

The unrest comes amid a political crisis that has pitted Maliki against several of his erstwhile government partners, less than three months before provincial elections.

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The violence broke four days of relative calm in Iraq following a spate of attacks claimed by Al-Qaeda's front group that killed at least 88 people on January 15-17, according to an AFP tally.

The militant group is widely seen as weaker than during the peak of Iraq's sectarian bloodshed, but is still capable of carrying out mass-casualty attacks on a regular basis.

The latest wave of violence meant the overall death toll from bloodshed in Iraq this month has already surpassed that of any of the previous three months, according to an AFP tally based on reports from security and medical officials.

Attacks in Iraq are down from their peak in 2006-2007, but they are still common across the country.