The bomb, which exploded on the highway from Balad to Samarra, also wounded 19 people, the sources said, adding that most of the casualties were Iranians.

It followed another car bomb against pilgrims travelling the same route on Monday that killed eight people and wounded at least 15.

In both cases, the pilgrims were on their way to a Shiite shrine in Samarra, which was bombed in February 2006, unleashing a wave of sectarian bloodletting in which tens of thousands of people died.

Iraq is home to some of the holiest sites in Shiite Islam and is visited by hundreds of thousands of foreign pilgrims each year, most of them from neighbouring Iran.

Sunni militants in Iraq, including those linked to Al-Qaeda, frequently attack Shiites, whom they regard as apostates.

Biden warns on violence in calls with Iraqi leaders
Washington (AFP) May 24, 2013 - US Vice President Joe Biden called Iraq's Prime Minister Nuri al-Maliki Friday amid anxiety over a rash of bombings and violence in the country and the situation in Syria, the White House said.

"The vice president expressed concern about the security situation in Iraq and pledged continued US support for Iraq in its fight against terrorism," said a statement from Biden's office.

A wave of bombings and attacks across Iraq has killed 420 people so far this month and has sparked fears of a return to widespread sectarian violence.

Biden also raised the importance of Maliki reaching out to leaders across the political spectrum, the White House said, and both men stressed the need for a negotiated settlement to end Syria's vicious civil war.

Washington has been piling pressure on Iraq to step up searches of Iranian flights routed over its territory heading for Syria.

US Secretary of State John Kerry had previously publicly accused Iraq of turning a blind eye to what Washington says are Iranian military shipments to the regime of Syrian President Bashar al-Assad.

On Thursday, Biden called Iraqi Kurdish leader Massud Barzani and Iraqi Council of Representatives speaker Osama Nujayfi.

Biden told Barzani that he commended a decision by Kurdish ministers and members of parliament to end a boycott and return to work in Baghdad.

Speaking to Nujayfi, a Sunni leader who has been at odds with Maliki, Biden "expressed concern about the security situation in Iraq, stressing the need for all of Iraq's political leadership to unequivocally renounce violence and seek to marginalize extremists," a statement said.

Biden managed the political side of the US withdrawal from Iraq and maintains contacts with the leaders of various factions in the country's fractious political system.

Violence in Iraq has fallen from its peak in 2006 and 2007 but attacks are still common, killing more than 200 people in each of the first five months of this year, according to AFP figures.