The US has urged North Korea to abandon any "provocative" plan to test-launch a long-range missile.
The Taepodong missile could reach Alaska (archive picture)
Secretary of State Condoleezza Rice said any launch would be viewed by the US as a "very serious matter".
Japan, Australia and New Zealand also gave warnings and South Korea urged the north not to put a "friend in danger".
North Korea has observed a self-imposed moratorium on missile launches since 1999, but is reported to have fuelled a new missile capable of reaching Alaska.
The missile, a Taepodong-2, is believed to have a range of up to 6,000km (3,700 miles).
Bad weather over the launch site on Tuesday suggested any launch would have to be delayed, while South Korean politicians said their government was still unsure about the threat.
"It is also very difficult to confirm whether the North will even fire the unidentified object or not," said Woo Sang-ho, a spokesman for the ruling Uri party.
Speaking at a news conference in Washington, Ms Rice expressed stern US opposition to any possible launch.
"It would be a very serious matter, and indeed a provocative act should North Korea decide to launch that missile," she said.
"We will obviously consult [with allies] on next steps, but I can assure everyone that it will be taken with utmost seriousness."
She stressed that North Korea had repeatedly agreed not to test-launch any new missiles, both independently and in the context of six-nation talks on its nuclear programme.
The US comments came hours after Japanese Prime Minister Junichiro Koizumi said Tokyo would take "stern measures" against any launch.
Australian Foreign Minister Alexander Downer also issued words of warning:
"North Korea would be gravely mistaken if it thinks that a missile test would improve its bargaining position in the six-party talks," he said.
Unnamed US officials have echoed reports in the New York Times on Sunday that North Korea had completed fuelling its new missile.
This suggests that Pyongyang could launch the missile within a month, officials said.
North Korea last tested a long-range missile in 1998, when it fired a Taepodong-1 missile, with a range of 2,000km (1,200 miles) over northern Japan.
The missile landed in the Pacific Ocean.
Speculation has been mounting for some time that the North was preparing for a test, after US satellites started noticing preparations at North Korea's Taepodong launch site at Musudan-ri.
Diplomats say that North Korean technicians are going through the same procedures undertaken before the test in 1998.
North Korea's every move is being monitored
Pyongyang has been silent on any possible launch, but in comments carried through its state news agency KCNA, it criticised the attitude of the US and Japan towards North Korea.
"The Korean army and people will do their best to increase the military deterrent with sharp vigilance to cope with the moves of the US, which is hell-bent on provocations for war of aggression on the DPRK (Democratic People's Republic of Korea)," the statement said.
The missile concerns follow months of stalemate on North Korea's nuclear ambitions.
Correspondents say North Korea may be using the missile threat to try to break the deadlock.