US Secretary of State Condoleezza Rice said today a missile launch by North Korea would be a "very serious matter" and regarded as a provocative act.
Rice was asked about a possible long-range missile launch by North Korea at a joint news conference with Spain's foreign minister.
"From our point of view it would be a very serious matter indeed," she told reporters.
North Korea is believed to have completed fuelling a missile capable of reaching Alaska, raising the probability of an imminent test launch.
US officials have watched with alarm as satellite photos showed launch preparations accelerating at the Musudan-ri missile facility in North Hamgyong province in North Korea's northeast.
The South Korean daily Dong-A Ilboquoted a Seoul government official as saying the launch could be imminent.
"We think North Korea has poured liquid fuel into the missile propellant built in the missile launching pad. It is at the finishing stage before launching" but the South Korean government did not know if fuelling was completed, he said.
The United States plans to join Japan in condemning any such test. Washington has warned Pyongyang against the launch in a message passed to North Korean diplomats at the United Nations, but there was no response, US officials said.
The officials 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 3,500 to 4,300 kilometres. 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 Centre for Non-proliferation Studies said in a recent report.
Nonetheless it would be Pyongyang's first test of a long-range missile since it stunned the world in August 1998 by firing a Taepodong-1 over Japan that landed in the Pacific Ocean.