The officials said they did not know the reason, but told Reuters possibilities include: Pyongyang has done this as a gesture to encourage negotiations with Washington; run into technical difficulties, or, more ominously, finished reprocessing fuel needed for a half dozen or more nuclear bombs.

"There's not much going on," one US official said when asked about current activity at Yongbyon.

Another said: "I sense there may be a pause in the action but would be nervous about concluding that for certain."

Meanwhile, Assistant Secretary of State James Kelly disclosed that Washington remains concerned that China, despite hosting six-party talks aimed at resolving the North Korean nuclear crisis, continues to abet Pyongyang's weapons programs.

China has been "quite restrictive" in stopping the flow of major technologies to the North but "there is some leakage around the edges," requiring Kelly to raise the issue with Beijing in the last two weeks, he told the Senate Foreign Relations Committee."I know that there have been examples in which China blocked shipments but... there are many North Korean companies and front companies operating around in China," Kelly said.

He gave no details but experts said China has aided the missile program of North Korea, among other countries, with dual use items and raw materials that are hard to control.

On reports of Yongbyon inactivity, Kelly would only answer questions in a closed session. The subject was dropped.

If Yongbyon operations have been deliberately halted to facilitate six-party negotiations, the effect may be offset by an ominous new development. US officials told Reuters on Wednesday the North used Russian technology to produce a new intermediate range ballistic missile that may be the most accurate and capable in Pyongyang's inventory.