The roadside bomb exploded when a convoy of Saraya al-Salam drove along a highway near Samarra, one source said, adding that several people were also wounded in the attack.

One of the eight militiamen killed was a commander who took part in anti-jihadist operations in Iraq, said another source.

Samarra -- a Sunni majority region -- is home to a major Iraqi security headquarters and to an important Shiite shrine where a 2006 bombing touched off two years of sectarian bloodletting.

The militia, led by Sadr whose political alliance triumphed in May 2018 parliamentary elections, has been deployed in the Samarra region since 2014 when the Islamic State group swept across Iraq.