Two NATO helicopter gunships violated Pakistan's air space in its northwest border area Monday morning. Two Nato helicopters violated Pakistans border limits on Monday morning, entering through the Torkham border located in Khyber Agency, said security officials. According to details, the helicopters penetrated into the Pakistani territory near Torkham and remained there for about five minutes before they returned to Afghanistan. No firings have been reported. Security officials are investigating the matter. Such incidents occurred several times in the last few months. Earlier this year, NATO helicopters entered the Pakistan's territorial sky, killing three Pakistani border soldiers while intending to attack some militant's hideouts on the Pakistani side. This incident has led to a closure by the Pakistani government of a border pass for NATO convoys supplying goods and fuel to the U.S.-led NATO troops stationed in Afghanistan via Pakistan, leaving hundreds of NATO supply trucks stranded on the way, falling prey to numerous terrorist attacks.