Ten Afghan civilians have been killed in a NATO airstrike in northeastern Afghanistan on Thursday.

The attack has caused outrage in Afghanistan and prompted a stern warning from President Hamid Karzai who has repeatedly condemned airstrikes that have mistakenly killed civilians.

Ten parliamentary campaign workers were killed in Thursday's attack. At least two people, a parliamentary candidate and a campaign worker, were also injured.

U.S. officials said the airstrike targeted a convoy which the military had monitored for a number of days. The target was groiup of insurgents.

NATO's ISAF said the matter was under investigation. "We're aware of the allegations that this strike caused civilian causalities and we'll do our best to get to the bottom of the accusations," U.S. Marine Corps Maj. Gen. David Garza, an ISAF official, said in a statement.

"What I can say is these vehicles were nowhere near a populated area and we're confident this strike hit only the targeted vehicle after days of tracking the occupants' activity."

The ISAF said it carried out "a precision airstrike targeting an Islamic Movement of Uzbekistan senior member," established to be the deputy shadow governor for Takhar.

"Intelligence tracked the insurgents travelling in a sedan on a series of remote roads in Rustaq district. After careful planning to ensure no civilians were present, coalition aircraft conducted a precision airstrike on one sedan and later followed with direct fire from an aerial platform. The vehicle was traveling as part of a six-car convoy, but no other vehicles were hit in the strike," the ISAF statement said.

ISAF said "initial reflections indicate eight to 12 insurgents were killed or injured in the strike, including a Taliban commander."