WASHINGTON (Kyodo) Six U.S. Democrats have urged Japan to suspend its plan to begin a test operation to extract plutonium at a nuclear-waste reprocessing facility in Rokkasho, Aomori Prefecture, Democratic congressional sources said Thursday.

"We firmly believe that the continued extraction of weapons-usable plutonium poses significant and unnecessary threats to international security and nonproliferation," the six lawmakers said in a letter sent to Japanese Ambassador to the U.S. Ryozo Kato.

"To that end, we are writing to encourage you to suspend plans to conduct active testing of Rokkasho in 2006 as part of a broader agreement to postpone operation of the Rokkasho reprocessing plant," says the letter, obtained by Kyodo News.

The six lawmakers include Edward Markey from Massachusetts, who has expertise in energy policy and nuclear nonproliferation, and Donald Payne from New Jersey.

While pointing out that Japan already has a plutonium stockpile of more than 40 tons, one of the sources said the test operation at Rokkasho goes against the trend of strengthening the global nonproliferation regimes and would have a negative impact on Iran's nuclear designs.

The lawmakers said they made the request to Japan "as part of a global initiative to reduce worldwide stockpiles of weapons-usable fissile materials -- highly enriched uranium and separated plutonium."

They also said the suspension "would promote nuclear disarmament and nonproliferation, and help prevent terrorists from acquiring nuclear weapons."

Japan Nuclear Fuel Ltd., which runs the reprocessing plant in the village, plans to start a test operation to extract plutonium by March so plutonium can be produced as early as spring.