US-North Korea ties have been strained in the past over missile tests.

The US State Department said Tuesday that North Korea's overall program of weapons of mass destruction violated UN resolutions, after President Donald Trump brushed off recent missile launches.

Choosing words carefully after Trump's latest warm words for North Korean leader Kim Jong Un, State Department spokeswoman Morgan Ortagus declined to say if North Korea's launches were of ballistic missiles or constituted weapons of mass destruction (WMD).

"I think the entire North Korean WMD program, it's in conflict with the UN Security (Council) resolutions," Ortagus told reporters.

She said the United States wanted a favorable relationship between Trump and Kim with a goal to "negotiate a peaceful end to the North Korean WMD program."

"We have said many times and we will continue to reiterate that the economic sanctions will remain in place until we're there," she said.

When he arrived in Japan for a weekend state visit, Trump tweeted that North Korea had tested "some small weapons" that had "disturbed some of my people, and others, but not me."

He appeared to be referring to his National Security Advisor John Bolton, who said Saturday there was "no doubt" that North Korea's launches had contravened United Nations Security Council resolutions.

North Korea wasted no time piling onto Bolton, who has long promoted a hawkish foreign policy, calling him a "human defect" and "war maniac."

North Korea has previously called for Secretary of State Mike Pompeo to be excluded from negotiations after he apparently pressed Trump at his February summit with Kim in Hanoi not to accept sanctions relief without further action by Pyongyang in ending its nuclear program.