Trump also said he hopes that U.S. Secretary of Defense James Mattis's upcoming visit to Seoul would set the framework for "close consultations".
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North Korea unleashed a spate of missile tests and conducted two atomic explosions a year ago, and the Kim regime is believed to already possess somewhere between 10 to 20 nuclear weapons.
Mattis will not demand the increase of payments for the United States military presence within his upcoming visits to Japan and South Korea, since they are not aimed at "major changes" in the USA course, the Kyodo news agency reported, citing its sources in the Japanese and the U.S. government.
"President Trump reiterated our ironclad commitment to defend the ROK [Republic of Korea], including through the provision of extended deterrence, using the full range of military capabilities", said the White House in a statement.
US President Donald Trump reaffirmed Washington's "ironclad commitment" towards South Korea's security and agreed to take steps to bolster joint defence capabilities, the media reported on Monday. But his new defense secretary is making South Korea and Japan his first stops overseas in his new role, signaling strong concern over Pyongyang's growing nuclear-weapons threat. At the time, then-candidate Trump threatened to pull out U.S. troop contingencies if the host countries did not pay for their deployment.
Analysts said Mattis' visit would also be a show of force to make sure the North Koreans know that the untested US administration won't waver.
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The THAAD is created to shoot down incoming missiles at an altitude of 40-150 km, but most of DPRK missiles targeting South Korea fly at an altitude of less than 40 km.
But Pyongyang showed clear technical progress past year with two underground nuclear explosions and some two dozen tests of short- to mid-range missiles, including one that flew successfully from a submarine.
Operations at the Yongbyon Nuclear Scientific Research Center could be running at a "considerable" level, said the website run by the U.S. -Korea Institute at Johns Hopkins University.
Video: North Korea: Credible Threat?
"We'll have to wait and see if Trump's "America First, ' doctrine is as incompatible to Obama's 'pivot" toward Asia as it seems", Ward said.
He'll be in Seoul on Thursday, and then visit Japan.