Two coordinated bombs targeting worshippers streaming out of joint Sunni-Shiite prayers in a mosque killed at least 30 people north of Baghdad on Friday, in Iraq's latest upsurge in violence.

Unrest elsewhere left three others dead, as authorities grapple with Iraq's worst bloodshed since 2008.

The country is stuck in a prolonged political deadlock while officials fear neighbouring Syria's 30-month civil war is increasingly spilling over across the border.

A roadside bomb hidden in a garbage bin went off outside Al-Salam mosque, a Sunni place of worship in the confessionally mixed city of Baquba, at around midday as Sunnis and Shiites left after a joint prayer session.

A second blast went off nearby a short time later, after onlookers and emergency responders arrived at the scene.

"When we finished the prayers, we went out to leave, and the suddenly there were explosions," said Khaled Najim, a day labourer who suffered wounds to his left leg.

"Everything was thrown into the air, people were shouting, I saw many victims."

Thirty people were killed and 24 wounded in the twin explosions, said an army major and Ahmed al-Azzawi, a doctor at the city's main hospital. Several nearby buildings were also damaged.

"I was praying when the first explosion happened," said 22-year-old Baraa Hamid, who was wounded in the stomach.

"I went out with my friends to evacuate the victims and the wounded, and then, a second explosion went off. I felt something hit me in my stomach, and then the police evacuated me."

Security forces imposed a tight cordon on the area and were searching nearby houses and buildings for the perpetrators.

No group claimed responsibility for the blasts, but Sunni militants linked to Al-Qaeda have claimed a spate of attacks in recent months in the capital and central Iraq.

They frequently target Shiite Muslims, whom they regard as apostates.

Baquba, about 60 kilometres (35 miles) north of Baghdad, and the surrounding province of Diyala are mostly populated by Sunni Arabs, although there are substantial Shiite Muslim and Kurdish minorities.

In 2012, Diyala had the highest per capita rate of violence in Iraq, according to Britain-based NGO Iraq Body Count. It remains one of Iraq's least stable areas and is regularly struck by deadly attacks.

Three car bombs in and around the city killed 10 people on Tuesday.

Another bombing targeted Friday prayers at a mosque in the town of Khanaqin, also in Diyala, killing one person, and a car bomb at a market north of Baquba wounded five.

In the restive northern province of Nineveh, a shooting and a bombing killed a soldier and a municipal official.

Iraq has seen a spike in violence that has already left more than 4,000 people dead this year, sparking fears it is slipping back into the all-out sectarian war that plagued it in 2006 and 2007.

Officials have sought to tackle the violence with high-profile operations targeting militants and tough traffic restrictions in the capital, but attacks continue to rock much of the country.

The authorities have vowed to press on with the campaign, which they say has led to the capture of hundreds of fighters and the killing of dozens more, as well as the dismantling of training camps and bomb-making sites.

But the government has faced criticism for not doing more to defuse anger in the Sunni Arab community over alleged ill-treatment at the hands of the Shiite-led authorities.

Analysts and diplomats say militant groups exploit that on the ground to recruit and carry out attacks.

The latest surge in violence comes as the government grapples with a prolonged political stalemate, with no significant legislation passed since March 2010 parliamentary elections.

Officials have also expressed concerns that Sunni militant groups operating in Syria, where a civil war since March 2011 has claimed more than 110,000 lives, are using Iraq to train and carry out attacks as well.