N. Korea Preparing For Another Nuke Test?

No sooner had an analysis of air samples taken by an Air Force plane over the Sea of Japan confirmed that the North Koreans had indeed set off a nuclear device, than U.S. intelligence picked up signs of activity at an underground test site, reports CBS News national security correspondent David Martin, raising the possibility that the North Koreans might test again.

"We're watching it, obviously, and discussing it with other parties, as well," said Secretary of State Condoleezza Rice. "I think it goes to say that it would further deepen the isolation of North Korea and I hope they would not take such a provocative act."

South Korea is aware of signs related to possible preparations for an additional North Korea nuclear test, Yonhap news agency reported, citing unnamed government officials. One official said various intelligence reports were coming in about a possible test, but that it was unclear how reliable they were.

Meanwhile, National Intelligence Director John Negroponte's office confirmed that North Korea conducted an underground nuclear explosion. In a short statement posted on its Web site, Negroponte's office confirmed that the size of the explosion was less than 1 kiloton, a comparatively small nuclear explosion. Each kiloton is equal to the force produced by 1,000 tons of TNT.

That's less than one-tenth the size of the atomic bomb the U.S. dropped on the Japanese city of Hiroshima in 1945, CBS News White House correspondent Mark Knoller reports. That could indicate that North Korea was unable to produce a bigger yield from its weapon.

"Analysis of air samples collected on October 11, 2006, detected radioactive debris which confirms that North Korea conducted an underground nuclear explosion in the vicinity of P'unggye on October 9, 2006," the statement said.

On Friday, a senior Bush administration official told The Associated Press that one test conducted on samples gathered after the detonation found a radioactive gas consistent with a nuclear explosion. At the time, however, U.S. intelligence was not ready to confirm that a nuclear test actually had taken place.

The statement from Negroponte's office provides the first official confirmation from the United States that a nuclear detonation took place, as Pyongyang has claimed.

Meanwhile, North Korea appeared to slip further into isolation, as China — under intense pressure to enforce new U.N. sanctions — inspected cargo trucks bound for its communist ally and stepped up construction of a border fence.

Japan — once a major trading partner with the North Korea — said it was considering further sanctions, and Australia banned the North's ships from its ports.

The Chinese inspections at a border crossing with the North came amid concerns that Beijing would ignore the new U.N. sanctions leveled against the reclusive communist country for its proclaimed nuclear test. China is a major trader with North Korea and its support is key to the success of the new U.N. measures, which call for nations to check cargo leaving and arriving from North Korea.

"Chinese action on cargo inspection appears to have been prompted as much by a fear of reprisal after the U.N. vote as it is by the mandate of the resolution itself," said CBS News foreign affairs analyst Pamela Falk.

"Chinese diplomats have made clear that they don't expect to participate in inspection of ship cargo, either as part of the voluntary U.N. provision, or as part of the U.S.-sponsored Proliferation Security Initiative," Falk reports, "but the inspection of land-based cargo has been occurring for some time as part of routine security and may increase."

China's customs agency and its commerce and foreign ministries refused to say whether the cargo checks were prompted by U.N. sanctions, but a top U.S. diplomat said the inspections were promising.

R. Nicholas Burns, undersecretary of state for political affairs, said there will be "enormous pressure on China to live up to their responsibility" in enforcing United Nations punishment of its ally, North Korea. "We are all banking on that."

But former State Department official Jack Pritchard says the sanctions so far are not enough to make the North Koreans regret testing, reports Martin.

"It's going to require some other measure of intervention to get the North Koreans to stop from what they're doing," Pritchard told Martin. Sanctions alone are certainly not going to work."

North Korea remained defiant, with its No. 2 ranking leader, Kim Yong Nam, saying the country would strengthen its military and "achieve a final victory in the historic standoff with the U.S." His televised remarks didn't touch directly on the sanctions.

U.S. officials were preparing a diplomatic swing through Asia to address divisions over how to impose the new sanctions. The measures, approved Saturday, also include an embargo on major weapons to Pyongyang and the freezing of the assets of businesses linked to the North's weapons programs.

The top U.S. envoy on North Korea's nuclear program, Christopher Hill, met on Monday with his Japanese counterpart, Kenichiro Sasae, and said the two agreed to implement the sanctions swiftly.

Hill told reporters in Tokyo that the common threat from North Korea has helped unite the regional powers, particularly China.

"I feel that we have a great deal of similar thinking with China. I think this nuclear test has brought China much closer to us," Hill said.

The U.S. diplomatic campaign was to continue Wednesday when Rice was to arrive in Japan before traveling to South Korea and China. She was expected to have a three-way meeting with the Japanese and South Korean foreign ministers Thursday in Seoul, Japanese officials said.

Meanwhile, Russia's top negotiator on the North Korean nuclear issues says Pyongyang may be willing to return to six-party talks on its nuclear weapons program, reports .

Amid the diplomacy, Iran — which has also attracted global criticism for its nuclear program — issued its first official reaction to the U.N. sanctions. The country's president, Mahmoud Ahmadinejad, rejected the American-initiated measures, accusing the United States of using the U.N. Security Council as a "weapon to impose its hegemony."

Japan has taken the hardest line against the North. On Friday, the Cabinet approved closing ports to North Korean ships and banning trade with the North.

Japanese Prime Minister Shinzo Abe told reporters Monday that his country was considering more sanctions that might be drawn up after it takes "into consideration actions by international society."

Australia announced that it would go beyond the U.N. resolution by banning the North's ships from entering its ports, except in dire emergencies.

"I think that will help Australia make a quite clear contribution to the United Nations sanctions regime," Foreign Minister Alexander Downer said.

China — North Korea's biggest trading partner — had balked at the cargo inspections, saying they would increase tensions.

But on Monday, customs inspectors examined cargo trucks bound for the North in the border city of Dandong. The officers opened the back of each truck and looked at its cargo, though they did not open individual containers.

Last week, reporters who visited the border post did not see inspectors open any trucks.

"The inspections are routine and conducted by quarantine officials," said Li Canhao, an officer at the Nanping crossing, in an eastern valley surrounded by mountains.

In a further sign of fraying ties between China and North Korea, the Chinese have been building a massive barbed wire and concrete fence along parts of its border with the North.

Although the project was approved in 2003, the fence-building appears to have picked up since the test was announced. Scores of soldiers have descended on farmland near the border-marking Yalu River to erect concrete barriers 8 to 15 feet tall and string barbed wire between them, farmers and visitors to the area said.

The sanctions should not cut off the flow of basic foodstuffs to the North, which has endured years of famine caused by bad harvests and poor economic policies.

But the U.N.'s food agency said Monday that millions of North Koreans face "real hardship" this winter due to reduced food aid from foreign donors.

Mike Huggins, a WFP spokesman who just returned from a five-day visit to North Korea, told reporters in Beijing, "If that food aid is not there, then there is going to be very real hardship."