The United States on Friday warned North Korea against conducting a "provocative" intercontinental missile test after U.S. officials said there were signs a test could take place as early as this weekend.

A test would be Pyongyang's first launch 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.

"Such a launch would be a provocative act and we would instead urge them to focus their energies and their activities on returning to the six-party talks," State Department spokesman Sean McCormack told reporters. He was referring to talks on curbing Pyongyang's nuclear program involving the United States, China, Russia, North and South Korea and Japan.

The launch -- expected to involve a Taepodong 2 missile with an estimated range of 3,500 to 4,300 km -- could come as early as this weekend, U.S. officials said.

They said it seemed increasingly likely Pyongyang would go through with the test -- rather than just making preparations to get U.S. and international attention -- but that it could still decide to cancel a launch.

U.S. officials have told Reuters they would not try to shoot down a test missile although McCormack told a news briefing "we will take necessary preparatory steps to track any potential activities and to protect ourselves."

American and Japanese "assets" -- including satellites and a U.S. guided missile ship -- have been moved into position to serve as long-range surveillance and tracking platforms.

The United States and its allies were caught off guard when Pyongyang last tested eight years ago and they are determined this time to be ready to gather critical intelligence on the North's capabilities.

McCormack said that in recent days Washington consulted key countries in a campaign to make clear to North Korea that "a missile launch would be a provocative act that is not in their interests and will further isolate them from the world."

A missile launch would be inconsistent with the 1999 moratorium declared by North Korean leader Kim Jong-il, which he reaffirmed in 2002, and with a Sept 19, 2005 statement in which Pyongyang and other parties to the six-country talks pledged to stabilize the Korean peninsula, he added.

The six-country talks are stalemated and international attention has shifted to concerns that Iran is building a nuclear weapon, which Tehran denies.

Some U.S. analysts believe North Korea, feeling ignored, would test to assert its importance or its pique over President George W. Bush's willingness to show flexibility to Iran even as he has held a tough line on Pyongyang.

U.S. officials said satellites recently spotted a missile on a flatbed truck at the Musudan-ri launch facility in North Hamgyong province in northeast North Korea. It was being readied for the launch pad, but fueling had not yet begun.

Reflecting growing congressional unease, two leading Democratic senators, Carl Levin of Michigan and Hillary Clinton of New York, urged Bush to develop a new strategy to address the North's nuclear and missile threats and appoint a senior envoy to direct the effort.

"We may be approaching the nightmare scenario in which our only option is to negotiate with a North Korea that can attack the United States with a nuclear weapon instead of a North Korea that is still working toward that capability," they said in a letter.