The US defended itself against charges its forces killed 11 Pakistani troops in an incident Wednesday that threatened to worsen already tense ties with its key war-on-terror ally. The US-led coalition in Afghanistan admitted carrying out an air strike in Pakistan Wednesday but said it targeted militants -- not the Pakistani soldiers -- and that its response was coordinated with Pakistan. The incident, the worst of its kind since Pakistan joined the "war on terror" in 2001, comes amid growing unease in Washington and Kabul over Pakistan's efforts to negotiate with Taliban militants in the border region.