U.S. Aircraft Keeps Watch for N. Korean Nuke Test

A U.S. military plane capable of detecting radiation was deployed over the Korean Peninsula amid concerns over a threatened nuclear test by North Korea, a U.S. official said Thursday.

The Kyodo News agency reported that the atmosphere detection plane — a WC-135W Constant Phoenix — took off from the U.S. air base at Kadena on the southern island of Okinawa.

The plane, which takes in atmospheric readings to measure radiation and other things in the air, is part of the U.S. effort to figure out if and when North Korea tests a nuclear weapon, the U.S. official said.

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

Col. Anne Morris of U.S. Forces Japan said she was not authorized to talk specifically about ongoing operations, but said U.S. forces were on alert for any moves by the North.

"Of course, everybody is being vigilant. But the U.S. position remains that we hope North Korea will allow the situation to be resolved diplomatically," she said.

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.

CountryWatch: North Korea

South Korean President Roh Moo-hyun 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.

Russian Foreign Minister Sergey 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, Poland. "We are working with the leadership of North Korea to stop steps that could negatively impact the situation."

Roh also ordered the South Korean 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 his 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.

Click here to go to FOXNews.com's North Korea Center.

The North's announcement also prompted outcry from 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.

The Associated Press contributed to this report.