North Korea appeared to slip further into isolation on Monday, 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 North Korea — said it was considering further sanctions, and Australia banned the North's ships from its ports.

The United States confirmed the underground explosion in North Korea last week was a nuclear blast, reporting that air samples gathered last week contained radioactive materials.

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 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.

At the United Nations, Chinese Ambassador Wang Guangya said, however, that his country will implement the U.N. sanctions and inspect cargo from North Korea for illegal weapons and missiles.

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But Wang indicated that China will not stop and board ships to search for equipment or material that can be used to make nuclear, chemical and biological weapons or ballistic missiles.

"This is a resolution we have to implement," Wang told reporters. "The question was raised whether China will do inspections. Inspections yes, but inspection is different then interdiction and interception. I think different countries will do it different ways."

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."

The office of National Intelligence Director John Negroponte confirmed that the explosion in North Korea had a force of less than 1 kiloton, a comparatively small nuclear detonation. Each kiloton is equal to the force produced by 1,000 tons of TNT.

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 did not 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.

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 Secretary of State Condoleezza 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.

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 U.S. 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."

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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 with North Korea, the Chinese have been building a barbed wire and concrete fence along parts of its 880-mile 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."