US Under Secretary of State Nicholas Burns sought to dismiss doubt over China's commitment to tough action against its communist neighbour, telling CNN: "The Chinese now are beginning to stop trucks at the 800-mile (1,400 km) border and inspect all of them."
China has made clear it is deeply concerned about the consequences of provoking a collapse of the impoverished and highly militarised state and its UN ambassador again clarified the limits of its action on Monday.
"Inspections yes, but inspections are different from interception and interdiction." Ambassador Wang Guangya told reporters, an apparent reference to stopping cargo at sea, which is one of the UN sanctions agreed to on Saturday.
Wang said the resolution did not make it mandatory for all nations to inspect cargo going to and from North Korea. He said states could carry out such an operation as necessary "in accordance with their national legal authorities."
Secretary of State Condoleezza Rice leaves for Tokyo, Seoul and Beijing on Tuesday to try to cement the unified UN approach and edge North Korea back toward six-party talks aimed at stopping its program to build a nuclear weapon.
The US government acknowledged that the Oct. 9 explosion, which prompted world-wide condemnation and the harsh sanctions regime, was a nuclear explosion as Pyongyang claimed.
"Analysis of air samples collected on October 11, 2006, detected radioactive debris which confirms that North Korea conducted an underground nuclear explosion," the director of national intelligence said in a statement.
WEB OF SANCTIONS
There were signs the new web of goods and weapons sanctions was starting to take effect. Australia said it was prohibiting all North Korean ships from entering Australian ports and Japan announced it was extending already harsh restrictions.
But Professor Nam Sung-wook of Korea University, an expert on North Korea, said the practical effect was questionable. "They (countries supporting sanctions) are in bed together but they're all dreaming different dreams," he said.
Most of Pyongyang's trade crosses through China, which has perhaps the most leverage on the reclusive state but fears a flood of refugees if the Pyongyang government collapses.
But it is not inclined to implement widespread inspections, said Xu Guangyu of the government-sponsored China Arms Control and Disarmament Association institute in Beijing.
He said of inspecting cargo for weapons-related material: "This is more a symbolic step than a real sanction measure."
"China just doesn't engage in that sort of trade with North Korea, so there's not much practical that needs to be done. It lets North Korea know our feelings." Weapons-related trade is a tiny fraction of the two countries' $1.5 billion trade.
'ASSERTIVE ACTION'
White House spokesman Tony Snow expressed confidence that China and South Korea would uphold the sanctions, noting they had never before committed themselves to such "assertive action" against Pyongyang.
"It is a significant factor not only diplomatically but they also understand the ramifications, and we are confident they will abide by it," he told reporters.
US analysts said this was crucial. "There is no way North Korea is going to negotiate seriously, if ever, as long as they know China and South Korea will bail them out," said Winston Lord, a former US ambassador to China.
The UN sanctions bar trade with North Korea in dangerous weapons, impose bans on heavy conventional weapons and luxury goods and ask nations to freeze funds connected with North Korea's non-conventional arms programs.
North Korea agreed in principle at six-party talks a year ago to scrap its nuclear arms programs in return for aid, security assurances and promises of better diplomatic ties.
But it has boycotted the talks among the two Koreas, Japan, the United States, China and Russia about implementing that deal for almost a year, soon after Washington imposed earlier financial sanctions over North Korea's alleged illicit trading activities.

Copyright Reuters, 2006