Washington: A defence official says that US Central Command will conduct its own investigation into NATO's involvement in airstrikes along the Afghan border that killed 24 Pakistani troops.

General James Mattis, who heads the command, is expected to appoint an investigating officer by Tuesday.

That's according to the defense official, who was not authorised to discuss the matter publicly and spoke on condition of anonymity.

NATO has a separate investigation under way. A key question to be examined by the US is who approved the airstrikes on Saturday and why.

Afghan officials say their soldiers called for help after being fired upon from the direction of Pakistani border posts.

Pakistani authorities claim the airstrikes were unprovoked.