The United States warned that a North Korean launch of a long-range missile would be a "provocative act" and began intensive diplomatic consultations on a response. "There are reports they may be preparing for a long-range missile launch," said Defense Department spokesman Bryan Whitman.
"The United States government as a whole has been consulting with allies in the region and has made clear than a North Korean missile launch would be a provocative act," he said.
Whitman noted that the United States has limited missile defenses but would not say whether it intends to use them against a North Korean missile launch.
However, he pointedly used the term "launch" rather than "test" to describe the North Korean preparations and said Pyongyang's intentions were not clear.
"A test would imply that you would know the intentions," he said. "We don't know the intentions."
Deputy Secretary of State Robert Zoellick said the United States was consulting with partners in the six-party nuclear talks with North Korea, as well as other nations.
China, South Korea, Russia, Japan and the United States are participants in the long-stalled talks with North Korea.
At the United Nations, US Ambassador John Bolton said he was consulting members of the Security Council "on what steps might be taken because it would obviously be very serious."
"Obviously the first preference is that the North Koreans not light the missile off. We've made that clear to them," he added.
In 1998, North Korea fired a two-stage Taepodong 1 missile over Japan into the Pacific Ocean, causing an international furore.
It declared a moratorium on flight tests of long range missiles in 1999 but said in 2005 that it would no longer keep to it.
The North Koreans appear to have completed fueling a long-range ballistic missile, greatly increasing the probability it will go ahead with its first test in eight years, the New York Times reported.
According to a senior US official quoted by the Times, satellite images suggest that booster rockets have been loaded onto a launch pad and liquid fuel tanks fitted to a missile at a site on North Korea's east coast.
As early as 2004, US intelligence reported that North Korea may be ready to flight test a Taepodong-2 missile capable of reaching the US mainland with a nuclear weapon-sized payload.
A two-stage Taepodong-2 missile could hit parts of the United States, while a three-stage Taepodong-2 could range all of North America, the head of the Defense Intelligence Agency told the US Senate in February 2005.
Previous unclassified Defense Department estimates date back to 1997, when a report put the Taepodong-2 missile's range at between 4,000 and 6,000 kilometers (2,500-3,750 miles) , and the Taepodong-1 at 1,500 km (940 miles).
The United States has been working feverishly, with mixed success, to field missile defenses capable of countering a limited missile attack by North Korea.
A North Korean launch would mark the first real test of the US system, which currently consists of an array of tracking and targeting radars and at least 11 interceptor missiles in silos in Alaska and California.
US Aegis warships have been modified for missile defense missions. Several are stationed in the western Pacific.
Their Spy-1 radars are capable of tracking missile launches. The US Missile Defense Agency also has been testing capabilities of warships to shoot down short and medium range missiles with interceptor missiles.
Last November, a US Navy cruiser intercepted a mock warhead after it separated from a medium-range missile in a test pover the Pacific.
