TOKYO >> U.S. national security adviser John Bolton on Saturday called a series of short-range missile tests by North Korea earlier this month a violation of U.N. Security Council resolutions and said sanctions must be kept in place.

The U.S., however, is willing to resume talks with North Korea at any time, Bolton said. Washington’s position on the North’s denuclearization is consistent and a repeated pattern of failures to rid North Korea of nuclear weapons should be stopped, he said.

Bolton was speaking to reporters in Tokyo ahead of President Donald Trump’s arrival for a four-day visit to Japan.

Bolton said that North Korea on May 4 and 9 tested short-range ballistic missiles, ending a pause in launches that began in late 2017. The tests are seen as a way of pressuring Washington to compromise without actually causing the negotiations to collapse.

“U.N. Security Council resolutions prohibit North Korea from firing any ballistic missiles,” Bolton said. “In terms of violating U.N. Security Council resolutions, there is no doubt about that.”

His comments came a day after North Korea’s official media said nuclear negotiations with Washington won’t 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 spokesman accused the U.S. of deliberately causing February’s collapse of talks between 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’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.