Nearly a million people descend on Karbala under tight security for the annual pilgrimage [Reuters]

Two car bombings within a span of four hours have killed eight people in Iraq's southern city of Karbala, where an annual pilgrimage is under way, police say.

In the first attack, which took place near a Shia shrine, police confirmed to Al Jazeera that 11 other people were injured when two vehicles packed with explosives detonated.

Al Jazeera's Rawya Rageh, reporting from Baghdad, said "the two car bombs were parked in a garage near a shrine and they went off in succession".

Only hours later, a second attack on a highway in the southern part of the city killed six people and injured 22 other.

The explosions come as thousands of Shia pilgrims make their way to Karbala to celebrate the birth of Imam Mahdi.

The annual pilgrimage in honour of one of the 12 imams revered by Shias, which has attracted at least a million people in the recent years, has often been target of attacks by extremists.

"Unlike previous years, Shia pilgrimages in recent months have largely gone by without incidents, most notably the pilgrimage to the Imam Kadhim shrine in Baghdad last month," our correspondent said.

In other violence, a US soldier was killed in southern Iraq, making it the fourth casualty for this month.

June was the deadliest month for US forces since 2008, with 14 soldiers killed.