The officials said the US has increased its surveillance over the isolated, communist country and has seen a North Korean missile launcher moving around, as well as construction of VIP seating in the eastern coastal city Wonsan.
The officials, who weren't authorised to discuss the matter publicly and spoke on condition of anonymity, said the new surveillance includes satellites, drones and other aircraft.
North Korea, which is banned by the UN from conducting long-range missile tests, says it is in the final stages of developing an intercontinental ballistic missile, fitted with a nuclear warhead, that could reach the US mainland. Experts say the North may be able to reach such capability in the next couple of years.
On Saturday, it conducted a ground test of a new type of high-thrust rocket engine, which it hailed as a breakthrough for the country's space program. Leader Kim Jong Un said the test was to determine the engine's thrust power, but weapons experts say such technology also has applications for the North's ballistic missile program.
The US and other countries have often accused North Korea of using its space program as a cover for military advances.
The US officials yesterday said it's unclear what type of missile launch may be coming. North Korea previously has conducted tests in Wonsan of its medium-range ballistic Musudan missile.
White House press secretary Sean Spicer yesterday described the North Korean threat as "grave and escalating."
He said the US is coordinating its strategy with allies.
Visiting the region last week, Secretary of State Rex Tillerson threatened North Korea with tougher sanctions, more pressure and possible military action.
The North responded by saying it isn't frightened by U.S. threats.
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