THAAD is a ground-based missile defense system created to shoot down short-, medium- and intermediate-range ballistic missiles.
Analysts say flight data from the North's second test, conducted Friday night, showed that a broader part of the mainland United States, including Los Angeles and Chicago, is now in range of Pyongyang's weapons.
The B-1B flight was in direct response to the missile test and the previous July 3 launch of the Hwansong-14 rocket, the USA statement said. It was estimated that Friday's missile could travel over 6,200 miles; meaning it could hit not only Alaska, but most of America's West Coast, and even the East Coast if fired in the other direction.
"As for the deployment of six (THAAD) launchers, South Korea and the USA will make a decision after consultations". Seoul also proposed to revise limits imposed by the US on the range and payload of South Korea's own missiles.
"We have a responsibility to our allies and our nation to showcase our unwavering commitment while planning for the worst-case scenario".
O'Shaughnessy described North Korea as an "urgent threat" - and said action would be taken if deemed necessary.
The presidential office also said that National Security Office (NSO) chief Chung Eui-yong had reported that a test was "imminent". It didn't specify exactly where the missile was stopped.
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"The ROK and the US had closely tracked the sign of a provocation through joint surveillance assets, and discussed the countermeasure in advance", MND spokesperson Moon Sang-gyun told media at a regular news briefing.
"The whole U.S. mainland" is now within reach, state media quoted North Korean leader Kim Jong Un as saying.
Had it been fired at a standard trajectory, the rocket could have reached Los Angeles, Denver and Chicago and would have even had NY and Boston within its sights, according to analysts. It does not target intercontinental ballistic missiles.
"The countermeasures - which were decided by Seoul and Washington after the North's missile launch - couldn't have been prepared if we didn't know the missile launch was going to happen", Blue House spokesperson Yoon Young-chan said in a written statement.
Trump blasted China for not doing more to help tackle the growing crisis yesterday in a Twitter rant.
"The U.N. Security Council has clear regulations on North Korea's launch activities that use ballistic missile technologies".
However, Trump said he was "very disappointed in China". We will no longer allow this to continue. "China could easily solve this problem!" the tweets read.
Moon noted that Seoul and Washington were expected to launch full-swing talks on additional THAAD launchers' deployment in the near future.