Plutonium reprocessing activity at a key North Korean (searchsite has apparently ceased, U.S. officials said Thursday.

It is unclear why the North Koreans stopped work at their reprocessing plant at Yongbyon (search), the officials said, speaking on the condition of anonymity.

Presumably, they either chose to stop or had technical problems at the plant. Unless something broke, the plant could be restarted at any time.

The plant turns spent nuclear fuel rods into plutonium that can be used in nuclear weapons. It's the only one the North Koreans are known to have. At the same site is a nuclear reactor that can make the spent fuel rods.

Also Thursday, officials said North Korea appears to be developing a new intermediate-range ballistic missile capable of reaching the United States. The missile hasn't been tested.

The North Korean move at its nuclear site in Yongbyon was first reported in Thursday editions of the Los Angeles Times.

The U.S. officials declined to say precisely when activity at the Yongbyon stopped, although other officials had said as recently as last month that low-level reprocessing was under way.

The North Koreans restarted the reactor at Yongbyon in late February. They are also thought to have accessed some 8,000 ready-to-reprocess spent fuel rods that had been in storage.

Reprocessing work may have begun sometime in the late spring or summer.

It is unclear whether North Korea could have reprocessed enough spent fuel to make a nuclear weapon. Washington estimates they already have at least one or two.

Experts had previously estimated that running the plant at full speed could make five or six new nuclear weapons out of the 8,000 rods at the rate of one a month. But officials had previously described the activity at Yongbyon as small in scale.

The North Koreans claimed they finished reprocessing the rods in April, but Western intelligence officials don't believe them.

Their new missile may have a range of 9,400 miles, a distance within the range of any U.S. state or territory, two U.S. government officials said.

Until now, the limit of North Korea's missile range was thought by U.S. defense experts to have been Alaska or Hawaii for heavier payloads and the western half of the continental United States for lighter payloads.

Whether Pyongyang could reach U.S. targets with a nuclear warhead is not clear; officials are not certain whether their nuclear weapons are small enough to fit on their missiles.

Some officials said the new missiles based on Russia's SS-N-6 "Serb," a Soviet-era, submarine-launched ballistic missile. This suggests cooperation, at a minimum, from Russian scientists or other entities, the officials said.

The administration has raised the issue with Russian government officials, who indicated surprise and disapproval of the activity, according to the U.S. officials.

However, other U.S. officials said the North Korean missile may be based on an indigenous design, rather than a Russian one.

North Korea's possession of missiles with a range covering almost half the planet could add a troubling dimension to its dispute with the United States over its nuclear weapons program.

In the absence of a proliferation agreement, the North could cap its missiles with nuclear warheads, leaving U.S. cities vulnerable to attack.

North Korea has maintained a moratorium on missile tests since 1998. But the country is believed to be obtaining information about the viability of its missiles from Iran, which has been receiving missile technology from North Korea and carrying out missile tests.

Two weeks ago, officials from North Korea and the United States, along with China, South Korea, Japan and Russia, met in Beijing to discuss ways of surmounting an impasse over the North's nuclear weapons programs.

State Department spokesman Richard Boucher said the administration hopes to deal with Pyongyang's missile program at a later stage of the talks. The discussions are expected to resume next month.

Boucher declined comment on the reported North Korean activity but cautioned Pyongyang against "any further provocative steps or difficulties."

"It's important for North Korea to think about moving in the right direction and not take steps in the wrong direction," Boucher said.

North Korea once again rejected the U.S. demand that it give up its nuclear weapons program, saying it has no intention of disarming itself completely.

In a commentary Friday, North Korea's official newspaper Rodong Sinmun also accused the United States of "seeking to mount a pre-emptive nuclear attack," according to the official KCNA news agency.

North Korea wants the United States to first sign a nonaggression treaty and normalize relations before it can feel safe enough to scrap its nuclear program. Washington says it could consider security guarantees and economic help if Pyongyang abandons the program first.