US national security adviser John Bolton has called a series of short-range missile tests by North Korea earlier this month a violation of UN Security Council resolutions and said sanctions must be kept in place.

Mr Bolton also defended the recent US seizure of a North Korean cargo ship, arguing Washington’s position on the North's denuclearisation is consistent and a repeated pattern of failures to rid North Korea of nuclear weapons should be stopped.

The US, however, is willing to resume talks with North Korea at any time, he added.

Mr Bolton was speaking to reporters in Tokyo on Saturday ahead of Donald Trump's arrival for a four-day visit to Japan.

He said North Korea tested short-range ballistic missiles on 4 and 9 May, ending a pause in launches that began in late 2017.

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The tests are seen as a way of pressuring Washington to compromise without actually causing the negotiations to collapse.

"UN Security Council resolutions prohibit North Korea from firing any ballistic missiles," Mr Bolton said. "In terms of violating UN Security Council resolutions, there is no doubt about that."

Mr Trump and other officials have played down the significance of the missile tests.

During his visit, Mr Trump and prime minister Shinzo Abe will "talk about making sure the integrity of the Security Council resolutions are maintained," he said.

The two leaders are also expected to discuss Iran, as well as trade and the bilateral security alliance, after playing golf and watching sumo wrestling on Sunday.

His comments came a day after North Korea's official media said nuclear negotiations with Washington will not resume unless the US abandons what Pyongyang describes as unilateral disarmament demands.

In a statement carried by the Korean Central News Agency, the North Korean spokesperson accused the US of deliberately causing February's collapse of talks between Mr Trump and North Korea's leader Kim Jong-un by making unilateral and impossible demands.

The North has also strongly protested the recent US seizure of a North Korean cargo ship that was involved in banned coal exports and demanded its immediate return.

Washington says the talks broke down because North Korea demanded sanctions relief in exchange for partially surrendering its nuclear capabilities.

Mr Bolton brushed off the North Korean rhetoric, saying, "I take much of what they say with a grain of salt."

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Calling the US seizure of the North Korean ship "appropriate," he said it may be a good time to discuss the return of the USS Pueblo, a naval intelligence ship held by the North since 1968.

Mr Bolton acknowledged the US has not been "hearing much from North Korea" since the Hanoi summit.

The US special envoy to North Korea, Stephen Biegun, "can't wait to talk to his North Korean counterpart but they haven't responded," he said, adding that Mr Biegun was "ready at any point to get on a plane and go anywhere."

Mr Trump's visit will largely highlight close ties with Mr Abe, who is now willing to hold a summit with Mr Kim without preconditions — a recent change from his long-held hawkish stance.

Mr Abe had said previously he will not meet Mr Kim unless the North takes concrete steps towards denuclearisation and resolve a decades-long dispute involving Japanese nationals abducted by North Korea.

Mr Bolton said he fully supports a possible Abe-Kim summit as an additional push towards resolving North Korea's missile and nuclear threats.

"Given Abe's willingness to hold this meeting with Kim Jong-un ... it would be certainly in North Korea's interest to accommodate the prime minister," he said.