North Korea is believed to have completed fueling a missile capable of reaching Alaska, raising the probability of an imminent test launch, U.S. officials said on Sunday.
The United States plans to join Japan in a sharp response if the test goes ahead.
Washington has warned Pyongyang against the launch in a message passed to North Korean diplomats at the United Nations but there was no response, American officials told Reuters.
The officials, speaking on condition of anonymity, said Pyongyang could still decide to scrap the launch, but that was unlikely given the complexity of siphoning fuel back out of a missile prepared for launch.
The test is expected to involve a Taepodong-2 missile with an estimated range of 2,175 to 2,670 miles (3,500 to 4,300 km). At that range, parts of Alaska in the United States would be within reach as well as Asia and Russia.
North Korea lacks an operational missile that can hit the continental United States, the California-based Center for Nonproliferation Studies said in a recent report.