Mansour was engaged in "specific actions, specific things ...In real time," Pentagon spokesman Capt Jeff Davis said here.
Asked if the Taliban threats were imminent, he said: "Yes, specific imminent threats to US and coalition personnel in Afghanistan." The Pentagon, however, did not give details and nature of the specific eminent threat.
He reiterated multiple times that the action against Mansour was a "defensive" strike.
While the US has been conducting similar defensive strike inside Afghanistan, this is probably for the first time that the US did a "defensive strike" inside Pakistan after the killing of Osama bin Laden.
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"This was a defensive strike against an individual who was actively engaged in planning and conducting operation that were targeting US and Coalition personnel," he said during an off camera news conference.
"This was considered a defensive strike. (But) the location (of the strike) required higher level of approval. Ultimately this was an individual who was specifically targeting US and coalition personnel and had specifically engaged in operations in the past that resulted in US and coalition personnel being killed," Davis said.
Asked if this has strained the US' relationship with Pakistan, he said, "I do not think so".
"We have a relationship with Pakistan. I think, it is overall a positive one. We work with them, to try and help them focus on their defensive needs most notably in fighting the Haqqani network which is active in the western areas and we are going to continue to do that," he said.
When asked if there was an effort to capture Mansour, he said, "no".
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Davis said the strike was "deliberately carried out in a place which was remote" and not near other things that would have raised the risk of a collateral damage.
According to the Pentagon spokesman "this was a US 4A mission" which is "unilateral" in nature.
"This is not a change in authorities at all. This is us continuing to do what we have been doing, which is to conduct strikes of defensive nature when we see anybody doing things of a threatening nature to US and coalition forces," he said.