**A Nato air strike has killed seven Afghan policemen in the north of the country, officials say.**The officers were mistakenly hit after a joint Nato-Afghan patrol was ambushed by Taliban insurgents, Afghan officials told news agencies.
The incident comes as Nato-led forces battle Taliban insurgents in a major operation in southern Helmand province.
On Sunday, 12 civilians died in that operation in a missile strike that Nato said had also killed three Taliban.