May 25, 2019, 3:03 AM UTC / Updated May 25, 2019, 4:56 AM UTC

TOKYO — U.S. national security adviser John Bolton has called a series of short-range missile tests by North Korea a violation of U.N. Security Council resolutions and says sanctions must be kept in place.

Bolton said Saturday in Tokyo that the U.S. position on the North's denuclearization is consistent and that a repeated pattern of failures to rid North Korea of nuclear weapons should be stopped.

His comment comes a day after North Korea's official media said nuclear negotiations with Washington won't resume unless the U.S. abandons what Pyongyang describes as unilateral disarmament demands.

In a statement carried by the Korean Central News Agency, the North Korean spokesman accused the U.S. of deliberately causing February's collapse of talks between U.S. President Donald Trump and North Korea's leader Kim Jong Un by making unilateral and impossible demands. The North has also strongly protested the recent U.S. 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 Korean demanded sanctions relief in exchange for partially surrendering its nuclear capabilities.

Bolton acknowledged the U.S. has not been "hearing much from North Korea" since the Hanoi summit. The U.S. 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 Biegun was ready to get on a plane and go "anywhere, any time."

Trump arrives in Tokyo later Saturday for a four-day visit largely highlighting close ties with Japanese Prime Minister Shinzo Abe.

Trump's visit will largely highlight close ties with Japanese Prime Minister Shinzo Abe, who is also willing to hold a summit with Kim without preconditions.

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

The two leaders are to discuss North Korea as well as trade, security, and tensions with Iran.