At least five Afghan police were wounded when NATO helicopters mistakenly struck their post Saturday, the force and Afghan police said.

One of the wounded was in a critical condition following the "friendly fire" in the central province of Ghazni in the early hours of the day, NATO's International Security Assistance Force (ISAF) said.

"Several Afghan National Police were wounded when two AH-64 Apache helicopters responded to small arms fire from insurgents in the early morning hours," it said in a statement.

Provincial police chief Khyal Baz Sherzai confirmed the incident but said six policemen were wounded in the raid in the province's troubled Andar district.

The police chief did not provide further details but said an investigation was underway.

More than 100,000 international troops, most of them deployed under the UN-mandated NATO force, are operating in Afghanistan to quell an insurgency which has reached its deadliest nearly eight years after the fall of the Taliban regime.

The surge in violence has raised concerns as the war-torn nation is preparing to vote on August 20 in historic presidential elections, the second-ever for the destitute nation.

The insurgents demanded Afghans boycott the vote and said they would block the roads leading to voting stations to prevent Afghans from casting their ballots.