The United States said on Thursday North Korea had demonstrated a "qualitative" improvement in its nuclear and missile capabilities after an unprecedented level of tests last year, showing the needed to sustain pressure on Pyongyang to bring it back to disarmament negotiations.
U.S. Deputy Secretary of State Antony Blinken told a joint news conference after a meeting with his Japanese and South Korean counterparts that North Korea had conducted 24 missile tests in the past year, as well as two nuclear tests, and learned from each one.
"Even a so-called failure is progress because ... they apply what they have learned to their technology and to the next test. And in our assessment, we have a qualitative improvement in their capabilities in the past year as a result of this unprecedented level of activity," he said.
"With every passing day the threat does get more acute," Blinken said, and referred to comments by North Korea's leader, Kim Jong Un, on Sunday that his country was close to test-launching an intercontinental ballistic missile (ICBM) of a kind that could someday hit the United States.
Blinken said it was vital for the United States, Japan, South Korea and other countries to boost cooperation to defend against the threat.
"At the same time, it's absolutely vitally important that we exercise sustained, comprehensive pressure on North Korea to get it to stop these programs, to come back to the negotiating table, and to engage in good faith on denuclearization," Blinken said, referring to international sanctions.
U.S. President-elect Donald Trump responded on Monday to Kim's comments on an ICBM test by declaring in a tweet that "It won't happen!"
Experts say preventing such a test is far easier said than done, and Trump gave no indication what new steps he might take to roll back North Korea's weapons programs after he takes office on Jan. 20, something successive U.S. administrations, both Democratic and Republican, have failed to do.
Former U.S. officials and other experts say the United States essentially had two options when it came to trying to curb North Korea's fast-expanding nuclear and missile programs - negotiate or take military action.
Neither path offers certain success and the military option is fraught with huge dangers, especially for Japan and South Korea, given their close proximity to North Korea.
Japanese Vice Foreign Minister Shinsuke Sugiyama said Tokyo was watching closely to see what kind of Asia policy Trump would follow, but did not expect major changes.
"Will it be exactly the same as we have it now? I doubt it. But basically, I don't see the direction as changing in a significant way," he told the news conference, adding that the U.S. security treaties with Tokyo and Seoul were an important pillar of U.S. policy.
Blinken said an effective sanctions campaign required "determination" and "patience." "I believe that as long as we sustain it and build on it, it will have an effect," he said.
In another tweet on Monday, Trump said North Korea's neighbor and only ally, China, was not helping to contain Pyongyang - despite Beijing's support for successive rounds of U.N. sanctions.
Blinken said Washington had seen positive signs from China in recent weeks in implementing new restrictions on coal imports from North Korea, but added: "That needs to be sustained ... to be carried forward."