An official spoke to Reuters, on the condition of anonymity, saying there had been indications a missile launch could be scheduled, but couldn't reveal any further intelligence.

That comes after a Russian lawmaker, who has just returned from Pyongyang last week, said preparations were underway.

"They are preparing for new tests of a long-range missile," Anton Morozov told Russian media. "They even gave us mathematical calculations that they believe prove that their missile can hit the west coast of the United States.

"As far as we understand, they intend to launch one more long-range missile in the near future and, in general, their mood is rather belligerent."