KABUL - Two Americans including a civilian were killed Wednesday when a gunman opened fire near a NATO base in Kabul Wednesday, the military coalition said, with Afghan officials calling it an "insider attack". No insurgent group has so far claimed responsibility for the attack, which highlights growing insecurity in Afghanistan as the Taliban step up their nationwide insurgency.  "One US service member and one US civilian died as a result of wounds sustained in Kabul today," NATO said in a statement, adding that three other Americans were wounded in the incident.