The attack took place in Abbasiya, around 15 kilometres (nine miles) north of Samarra, on Monday evening and completely destroyed two houses, the sources said. Thirteen people were also wounded.
"The destruction was huge. Those homes were by the (Tigris) river and had been requisitioned by the militiamen to use as an observation post," the police officer said.
He said the militiamen were monitoring the river to prevent Islamic State jihadist fighters coming from the east from crossing over to the west bank.
Samarra is a mainly Sunni city that is home to one of the holiest Shiite Muslim sites in Iraq which is guarded by large contingents of Shiite militiamen.