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.