The Trump administration has warned that all options, including a military strike, are on the table to block North Korea from developing a nuclear-tipped missile that could reach the USA mainland.
The rare White House briefing comes as top USA and military officials grow more concerned over an increase in nuclear tests out of North Korea.
"A WILD DREAM" Earlier on Wednesday, North Korea's foreign ministry called US attempts to make Pyongyang give up its nuclear weapons through military threats and sanctions "a wild dream" and like "sweeping the sea with a broom".
North Korea is making steady progress toward developing ballistic missiles that can hit the USA and its capabilities will match its rhetoric if left unchecked, the commander of US forces in South Korea said. "We will never accept North Korea as a legitimate nuclear weapons power", he tweeted.
The top U.S. commander in the Asia-Pacific, Admiral Harry Harris, told the U.S. Congress that the Terminal High Altitude Area Defense (THAAD) system would be operational "in coming days".
China has been urging restraint by both Pyongyang and Washington.
North Korea has carried out a series of missile launches in recent months and the USA has responded by deploying an aircraft carrier group to the region.
Meanwhile, US senators have arrived in Washington for an extraordinary briefing on the threat posed by North Korea.
United States forces have begun deploying an anti-missile system in South Korea that has triggered anger from China.
South Korea said in a statement Wednesday that unspecified parts of THAAD were deployed.
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China's envoy on North Korea, Wu Dawei, met his Japanese counterpart, Kenji Kanasugi, for talks in Tokyo and they agreed that they would "respond firmly" to any further North Korean provocation, Japan's foreign ministry said.
Adm Harris said he believed that North Korea would try to attack the United States as soon as it had the military capabilities. "We remain open to negotiations towards that goal", it said in a joint statement by Secretary of State Rex Tillerson, Defense Secretary Jim Mattis and Director of National Intelligence Dan Coats.
"The weapons that North Korea would put against the Carl Vinson strike group are easily defended by the capabilities resident in that strike group", Harris said. It said it would win in a "death-defying struggle against the USA imperialists".
While Harris wouldn't discuss publicly scenarios for a pre-emptive strike against North Korea, he did address the risk that US action could be met by a response that would kill many South Koreans, Japanese and USA troops in the region.
But the system's installation is also upsetting China, the country Trump hopes to work with to rid the North of nuclear weapons.
However, the move has angered China, which said the advanced system would do little to deter the North, while destabilizing the regional security balance.
"The United States seeks stability and the peaceful denuclearization of the Korean peninsula", they wrote.
The Pentagon said the deployment was a critical measure to defend South Korea and its allies against North Korean missile threats and it would complete it "as soon as feasible".
Over the past year North Korea's missile program has apparently progressed, despite some spectacular launch failures involving intermediate range mobile launched missiles.