But this would require UN member countries to view the North Korean threat as serious and to be united in a willingness to absorb the economic and political effects of sanctions, the former director of the Office of Foreign Assets Control said.

Also, there is a "need for full cooperation in all aspects of the program including the interception and inspection of cargo on North Korean ships," Newcomb said.

He did not name a specific country but during a meeting with President George W. Bush at an Asia-Pacific summit on Saturday, South Korean President Roh Moo-hyun stopped short of pledging Seoul's full participation in a US plan to intercept North Korean ships suspected of carrying arms.

US officials and experts are also concerned about the extent to which China plans to implement the sanctions.

Newcomb stressed the need for regular monitoring of sanctions by senior leaders to "ensure every nation is doing its part and its actions are coordinated."

Daniel Poneman, a top nonproliferation official in President Bill Clinton's White House, said sanctions can be a useful tool in US diplomacy and the conditions outlined by Newcomb are essential.

But he was more pessimistic about US diplomacy succeeding in this case.

Newcomb's conditions have not been met and Pyongyang has not been presented by the United States with a clear package of inducements - if it gives up its nuclear weapons - and penalties - if it does not, Poneman said.

After a year-long boycott, North Korea recently said it would return to six-country talks on its nuclear program but so far a date has not been set.

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