WASHINGTON: US forces killed Taliban leader Mullah Akhtar Mansour in an air strike last week because he was engaged in plotting that posed "specific, imminent threats" to US and coalition troops in Afghanistan, the Pentagon said on Monday.

The drone attack against Mansour just inside Pakistan on Saturday was carried out under US rules of engagement that allow US forces to conduct defensive strikes against people engaged in activity threatening US and coalition personnel, said Navy Captain Jeff Davis, a Pentagon spokesman.

President Barack Obama approved the strike on Mansour based on a law authorizing military force in Afghanistan following the Sept. 11 attacks, Pentagon officials said. Since the strike was conducted in Pakistan, it required presidential authorization.

Davis told reporters it was the first time he was aware of that the US military had conducted an attack inside Pakistan under the Pentagon's rules of engagement governing defensive strikes.

Other strikes inside Pakistan, including one that killed al Qaeda leader Osama bin Laden in 2011, have been based on rules authorizing counterterrorism operations, a Pentagon official said.

"This was considered a defensive strike and given the location (Pakistan) required a higher level of approval," Davis told a briefing. Pakistan has called the strike a violation of its sovereignty.

"This (Mansour) 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.

Pressed on whether Mansour had simply been engaged in general plotting or if there was something more specific in the works, Davis said the Taliban chief had been plotting "specific actions, specific things ... in real time."

Asked if the threats were imminent, he said: "Yes, specific imminent threats to US and coalition personnel ... in Afghanistan."

The White House has interpreted the law passed after the 2001 attacks as granting the military the legal authority it needs to conduct air strikes against the Taliban and other groups under certain conditions.

Davis avoided saying whether Washington had notified Pakistan before the attack, but he said the two sides have an ongoing dialogue and had discussed Mansour in the past.

"We have ongoing discussions with them about people we're targeting, to include this individual, and we've had conversations with them both before and after," Davis said.

Davis said the military can carry out defensive strikes against anyone directly threatening US or coalition personnel.

They also can carry out so-called "in extremis" strikes to prevent attackers from overrunning Afghan forces or capturing key terrain, he said. And US forces have the authority to strike al Qaeda and its remnants in Afghanistan in what are deemed counterterrorism operations.