The US has readied missile defence systems in Japan following North Korea's announcement of a planned rocket launch, ostensibly to deploy an earth observation satellite.
North Korea has notified UN agencies that it will launch the rocket between 8-25 February, which has triggered international opposition from some governments that see it as a long-range missile test.
A US Navy spokesman has confirmed that the missile tracking ship USNS Howard O. Lorenzen arrived in Japan this week but has declined to say if it was in response to the North's plans.
The launch comes soon after North Korea's fourth nuclear test last month and concerns have been raised that it plans to fit nuclear warheads on its missiles, giving it the capability to strike South Korea, Japan and potentially even the West Coast of the US.
Pentagon officials have reportedly said that satellite footage of the lift-off site shows the fuelling of the rocket has already begun, citing increased activity around the missile launch and fuel storage areas. The preparations are advanced enough that a launch could be completed within "a number of days".
"We will, as we always do, watch carefully if there's a launch, track the launch, [and] have our missile defence assets positioned and ready," said US Defence Secretary Ashton Carter.
"We plan a lot about it. We and our close allies - the Japanese and the South Koreans - are ready for it."
Boosters and other expendable parts of the rocket will be tracked as they fall back to earth in the hope they can be retrieved and analysed for clues on Pyongyang's rocket programme.
"Retrieving parts or objects from the launch vehicle are the most important part of the rocket analysis," said Markus Schiller, a rocketry expert based in Germany.