The White House has said that the US is "fully capable" of defending itself against a North Korean ballistic missile attack.
Asked whether North Korea's threat was being taken seriously, a White House spokeswoman said that foreign government's must be "taken at their word".
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MPs say there must be no way back for the former London mayor as he hints he could seek to rejoin the party.
President Donald Trump has requested that the Justice Department investigates whether the FBI infiltrated his campaign.
High pressure will help keep conditions relatively quiet through the course of this week.