The suicide attacker rammed his explosives-laden vehicle into a gathering of Shiite fighters around 20 kilometres (12 miles) south of Tikrit.
An army lieutenant colonel said nine of the Shiite volunteers were killed and 13 other people were wounded, including five civilians.
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A medical source in Samarra, further south, confirmed the toll.
The past few days have seen a surge in violence south of Tikrit and around Samarra between pro-government forces and the Islamic State (IS) jihadist group.
Joint operations by the army, Kurdish peshmerga fighters and Shiite militia groups have, with Iranian backing, squeezed IS fighters out of some of their positions in eastern Iraq.
The jihadists, who hold Tikrit and have positions around Samarra, have launched numerous attacks around both cities to maintain pressure on pro-government forces.