A bomb has exploded as Sunni worshippers left a mosque in west Baghdad, one of two attacks that have killed seven people in Iraq, officials have said.

Police said the bomb left by the side of the road at the Omar mosque killed four people and wounded 11 others as they were walking away after Friday prayers.

In a separate incident, police said that two carloads of gunmen attacked a security checkpoint in Fallujah early on Friday, killing three policemen.

Two other policemen were also wounded in the attack in the city, considered a former al-Qaeda stronghold 65km west of Baghdad.

Health workers confirmed the casualties. All officials spoke on condition of anonymity because they were not authorised to talk to media.

Security forces are struggling to contain the country's most relentless round of violence since the 2011 US military withdrawal.

Sparking fears

More than 30 Sunni mosques have been hit in the past two months and over 100 worshippers killed, in addition to attacks on Shia neighbourhoods, security forces and other targets.

The surge of violence has sparked fears that the country could spiral into a new round of widespread sectarian bloodshed similar to that which brought the country to the edge of civil war in 2006 to 2007.

Tension stemming from months of protests by Iraq's Sunni minority against the mainly Shia-led government, many of whom feel they have been marginalised and unfairly treated since the 2003 US-led invasion, escalated sharply last month after a deadly crackdown by security forces on a Sunni protest camp.

In a show of national unity, Shia and Sunni worshippers held joint prayers Friday in a Shia mosque in central Baghdad amid tight security measures. Worshippers chanted: "Shias and Sunnis are brothers."

Sunni cleric Khalid al-Mulla urged the government and the Iraqi people to stop the bloodshed by uniting against "the terrorists who want to kill our sons in the name of Islam".

Sunni fighters, including al-Qaeda, have long targeted Iraq's Shia majority and government security forces.

But the attacks on Sunnis mosques have raised the possibility that Shia fighters are also growing more active.

"The blood that is being shed is precious Iraqi blood ... so we should say `Enough' to these killings," Mulla said.

Source: AP