British Foreign Secretary Boris Johnson on Saturday blamed North Korea for the stand-off with the United States and said London wants a diplomatic end to the crisis.

"The North Korean regime is the cause of this problem and they must fix it," Johnson wrote on Twitter.

"The international community is shoulder to shoulder in ensuring North Korea stops its aggressive acts.

"We are working with the US and our partners in the region to bring this crisis to a diplomatic end."

US President Donald Trump on Saturday reassured Guam it was secure amid mounting regional tensions, vowing that American military forces "stand ready" to safeguard the US Pacific island territory against a belligerent North Korea.

The North has threatened to fire ballistic missiles over Japan towards the tourism-dependent island, as Pyongyang and Washington ratchet up their war of words.

Chinese leader Xi Jinping urged Trump to avoid rhetoric that could inflame the situation, while French President Emmanuel Macron warned against an "escalation of tensions".