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South Korean President Roh Moo-hyun has ordered his government to send a "grave warning" to North Korea about the consequences of a threatened nuclear weapons test, Yonhap news agency reported Thursday.

Meanwhile, a U.S. military plane capable of detecting radiation has taken off from southern Japan, believed to be part of U.S. efforts to monitor for signs of a possible nuclear test by North Korea, a news report said Thursday.

Russian Foreign Minister Sergei Lavrov said that Moscow was working with Pyongyang to try to dissuade it from a test.

"We must do everything so that that doesn't happen," Lavrov said at a news conference on a visit to Warsaw. "We are working with the leadership of North Korea to stop steps that could negatively impact the situation."

Roh also ordered the government to draw up a "contingency plan" if the nuclear standoff with North Korea worsens, Yonhap said, citing unidentified presidential staff.

At the same time, Roh instructed the South Korean government to step up diplomatic efforts to forestall a North Korean test, the report said.

Roh's orders came after a meeting with his top security adviser, according to Yonhap.

Calls to the presidential office went unanswered on the first day of a three-day holiday.

North Korea threatened Tuesday to conduct a nuclear test to prove the country is a nuclear power. Pyongyang claims it has nuclear weapons and needs them to deter a U.S. attack, but hasn't performed any known test to verify that.

The North's announcement prompted outcry from a host of nations including China, the North's main ally. Beijing's ambassador to the United Nations urged Pyongyang Wednesday not to go ahead with a test, warning of "serious consequences."

Wang Guangya said at the U.N. that "no one is going to protect" North Korea, if it goes ahead with "bad behavior."

"I think if North Koreans do have the nuclear test, I think that they have to realize that they will face serious consequences," Wang said Wednesday.

The comment was China's most forceful public response yet to its ally's announcement Tuesday, and a break with Beijing's usual conciliatory strategy of avoiding warnings to or criticism of the North.

Beijing the North's main source of food and fuel aid had previously appealed for restraint but hasn't said what it might do if Pyongyang detonates a bomb.

The rebuke could spell trouble for North Korea, which faces a relatively united front against its nuclear aspirations, in sharp contrast to the fractured reaction to a series of North Korean missile tests in July. At that time, China accused Japan of overreacting in calling for sanctions.

Earlier Thursday, a pro-North Korean newspaper based in Japan warned that Pyongyang was not bluffing.

"The nuclear test statement was not empty language, but announced on the premise of action," the Choson Sinbo said. "Carrying out a nuclear test is an inevitable conclusion ... under a condition where (the country) declared possession of nuclear weapons in February last year."

The paper, run by an association of North Korean residents in Japan, is not part of the North's official media but is considered one of its propaganda tools. Its articles are believed to reflect the country's position.

It hedged its warning by saying the crisis can be overcome if the U.S. begins to take action toward denuclearization of the Korean Peninsula with "the same goal" of North Korea.

In his first reaction to the North's announcement, Roh called Wednesday for a "cool-headed and stern" response and ordered his government to let the North know what the consequences would be if it goes ahead with a test.