The US military expects another North Korean missile launch in the next few days, American defence 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 said.
The new surveillance includes satellites, drones and other aircraft, the officials added.
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.
Earlier this month, Pyongyang fired off four ballistic missiles into the Sea of Japan, reportedly reaching within 200 kilometres (120 miles) of Japan's shoreline.
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 programme.
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 programme.
The US and other countries have often accused North Korea of using its space programme as a cover for military advances.
The US officials said it is unclear what type of missile launch may be coming.
North Korea has previously conducted tests in Wonsan of its medium-range ballistic Musudan missile.
North Korea has emerged as perhaps the Trump administration's most pressing national security concern, and the latest surveillance information comes as the US explores new diplomatic, security and economic measures to halt Pyongyang's nuclear and ballistic missile programmes.
White House press secretary Sean Spicer on Tuesday described the North Korean threat as "grave and escalating".
He said the US is co-ordinating 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 is not frightened by US threats.