For three years, the Bush administration has waged a campaign to choke off North Korea's access to the world's financial system, where U.S. officials say the nation launders money from criminal enterprises to fuel its trade in missile technology and its efforts to build a nuclear arsenal.

That effort has started to pay off.

U.S. pressure forced Macao this year to freeze North Korean assets in one of its banks, then foiled North Korea's panicky attempts to find friendly bankers in Vietnam, Mongolia, Singapore and Europe. And after North Korea's Oct. 9 nuclear test, China ordered some of its major banks to cease financial transactions with the country.

The cash crunch appears to have played a key role in North Korea's decision Tuesday to return to six-nation talks over its nuclear ambitions. North Korean officials said that as part of the talks, they wanted to raise the issue of lifting financial sanctions.

"They're not coming back because they want to give up nuclear weapons," said David L. Asher, the U.S. State Department's point man on North Korea until last year. "They are feeling the financial pressure and the cutoff from the international financial system, so they are trying to make nice."

North Korea continues to have access to banks in both countries, according to current and former U.S. officials who say that without those nations' cooperation, the U.S. effort will be largely ineffective.

Both major powers have historically been more concerned about protecting their strategic interests than in joining U.S. efforts to sanction their neighbor.

Stuart Levey, the U.S. Treasury undersecretary in charge of investigating terrorist financial webs, has traveled to Russia and China, including a trip to Moscow last week. Levey said he had "constructive discussions" with his Russian counterparts, but declined to say whether they would act. A Russian Foreign Ministry official said they had "agreed to further cooperation."

U.S. officials believe that both countries will continue to resist American appeals for a further crackdown in part because of their "historic ties to North Korea," said a senior counter-terrorism official, who spoke about the U.S. campaign on condition of anonymity.