"It would be a very serious matter and, indeed, a provocative act should North Korea decide to launch that missile," Rice said amid indications that the North Koreans could launch an intercontinental ballistic missile at any moment.
The senior U.S. diplomat said the United States would talk to other nations about action should the North go ahead, and "I can assure everyone that it would be taken with utmost seriousness."
President Bush briefly discussed the missile test with Russian President Vladimir Putin during an 18-minute phone call the Russian official placed to the American president on Monday. The leaders plan to remain in touch on the missile issue, said Kate Starr, a spokeswoman for the National Security Council.
The United States, Japan, Australia, South Korea and other countries have urged North Korea to abandon any missile firing, but there was no sign of backing down. U.S. officials said Monday the missile was apparently fully assembled and fueled, giving the North a launch window of about a month.
Unlike other preparatory steps the United States has tracked, the fueling process is very difficult to reverse, and most likely means the test will go ahead, one senior administration official said.
The precise timing is unclear, the official said.
At U.N. headquarters in New York, U.S. Ambassador John Bolton said he was holding preliminary consultations with Security Council members on possible action if North Korea fires the missile, thought to be a three-stage Taepodong-2 with a non-nuclear payload.
"But we don't really know what the North Korean intentions are at this point, so I think we need to wait for the event," Bolton said.
The U.S. ambassador to Japan, Thomas Schieffer, said economic sanctions were an option.
"I think sanctions would have to be considered, but I wouldn't want to describe what actions we might take," Schieffer said through a U.S. Embassy official.
U.S. officials have said the 116-foot-long missile has a firing range of 9,300 miles and could reach as far as the U.S. West Coast. Most analysts, however, say North Korea is still a long way from perfecting technology that would make the missile accurate and capable of carrying a nuclear payload.
The North's missile program has been a major security concern in the region, adding to worries about a pursuit of nuclear bombs. North Korea shocked its neighbors when it test-fired an earlier missile version over northern Japan in 1998.
Japan says a new launch would threaten Japanese security and violate an agreement North Korea signed in 2002 and reaffirmed in 2004. Rice said it would also end a self-imposed moratorium on test firings that North Korea has observed since 1999 and a disarmament bargain it struck with the United States and other powers last year.
Rice said a launch now "would once again show North Korea is determined to deepen its isolation, determined not to take a path that is a path of compromise and a path of peace, but rather instead to once again saber-rattle."
The reports of a potential launch came during a prolonged hiatus in nuclear disarmament talks among North and South Korea, China, Japan, the United States and Russia. The earlier disarmament deal gives the North economic rewards for giving up weapons.
The agreement faltered almost as soon as it was announced in September 2005, and the North later walked away from talks in a dispute over a U.S. crackdown on alleged North Korean counterfeiting and fraud.
There have been no talks since last November.
North Korea says it needs nuclear weapons and a delivery system to counter what it contends are U.S. intentions to invade or topple the government. The United States has repeatedly denied any plans to invade.
U.S. intelligence indicates that the missile was fueled in recent days, said two officials, who requested anonymity because the information comes from sensitive intelligence methods.
The United States assumes North Korea would perform a test, not fire the weapon as an act of war, and could claim afterward that it was launching a space mission, one official said. That would still be considered a violation of the moratorium North Korea has observed since 1999, the official said.
The test would probably take place over water and occur during daylight hours, the official said. The United States would probably know almost instantly.
North Korea is 14 hours ahead of the U.S. East Coast.
Possible reasons for a test now include North Korean anger over a U.S. financial crackdown, which the North calls a form of sanctions, or an attempt to demonstrate that it has more technical prowess than Iran and should get at least as good a deal from the world as Iran has been offered to give up disputed nuclear activities.
North Korea referred to its missile program for the first time Monday, but has not said it intends to perform the test.
A North Korean state television broadcast, monitored in Seoul, South Korea, cited a Russian editorial on the missile and said the North "has the due right to have a missile that can immediately halt the United States' reckless aerial espionage activity."
The North has repeatedly complained in recent weeks about alleged U.S. spy planes watching its activities.