GHAZNI, Afghanistan, April 4 (Reuters) - Gunfire from a helicopter operated by NATO forces killed four Afghan police officers in the eastern province of Ghazni, a district police chief said on Thursday.

Civilian casualties are a source of friction between President Hamid Karzai and his international allies, and the mistaken killing of members of the Afghan security forces is likely to compound Afghan government anger.

The four Afghan Local Police (ALP) officers were in a village in Deyak district when the helicopter fired on them on Wednesday, said district police chief Faiz Mohammad.

"The gunship must have mistaken the policemen for insurgents," police chief Faiz Mohammad told Reuters, adding that the four were not wearing uniforms. Two civilians who were nearby were wounded, he said.

A spokesman for the NATO-led International Security Assistance Force said it was "assessing" the report.

"We're aware of local reports of an alleged air strike in Ghazni province yesterday in which several people were reportedly killed," the spokesman said.

In recent years, Deyak district has largely been under the control of the Taliban but the ALP, who are often recruited from militias, have pushed the militants out of the area.

Last week, an air strike by NATO helicopter supporting Afghan security forces killed two children and nine suspected Taliban in a different area of Ghazni province. {ID nL3N0CM02D}

(Reporting by Mustafa Andalib; Writing by Katharine Houreld and Hamid Shalizi; Editing by Robert Birsel)