A bomb exploded amid Sunni worshippers leaving a mosque in west Baghdad, one of two attacks Friday that left seven dead in Iraq, officials said.

Iraqi security forces are struggling to contain the country's most relentless round of violence since the 2011 U.S. military withdrawal. Over 30 Sunni mosques have been hit in the past two months and over 100 worshippers killed, in addition to attacks on Shiite neighborhoods, security forces, and other targets.

Police officials 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.

Meanwhile, police said that two carloads of gunmen attacked a security checkpoint in the city of Fallujah early Friday policemen, killing three. Two other policemen were also wounded in the attack in Fallujah, a former al Qaeda stronghold 40 miles west of Baghdad.

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

The surge of attacks 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 2006-2007. Tension stemming from months of protests against the Shiite-led government by Iraq's Sunni minority, many of whom feel they've been marginalized and unfairly treated since the 2003 U.S.-led invasion, escalated sharply last month after a deadly crackdown by security forces on a Sunni protest camp.

In a show of national unity, Shiite and Sunni worshippers held joint prayers Friday in a Shiite mosque in downtown Baghdad amid tight security measures.

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 militants, including al Qaeda, have long targeted Iraq's Shiite majority and government security forces. But the attacks on Sunnis mosques have raised the possibility that Shiite militias are also growing more active.

"The blood that is being shed is precious Iraqi blood ... so we should say `Enough' to these killings," al-Mulla said. Worshippers chanted, "Shiites and Sunnis are brothers."