NATO helicopters based in Afghanistan carried out at least two airstrikes in Pakistan that killed more than 50 militants after the insurgents attacked a small Afghan security outpost near the border, spokesmen said Monday.

NATO justified the strikes based on "the right of self-defense." Pakistan is sensitive about attacks on its territory, but U.S. officials have said they have an agreement that allows aircraft to cross a few miles into Pakistani airspace if they are in hot pursuit of a target.

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