TOKYO (Reuters) - Japan’s top government spokesman on Monday said he hopes Japan and South Korea study the North Korean issue “with rigor” as tensions with Pyongyang rise after it conducted a series of weapons tests and a war of words with the United States.

U.S. special envoy for North Korea, Stephen Biegun, in Seoul for talks with officials prior to visiting Japan, said Washington is willing to discuss “all issues of interest” and that the United States wants to reopen negotiations with Pyongyang.