Robert Einhorn, the State Department's special adviser for nonproliferation and arms control, made Seoul a first stop on his Asian tour to encourage allies to act aggressively to enforce sanctions against North Korea and Iran, nations he said constitute "two of the greatest threats" to nuclear nonproliferation and international security.
He met Monday with South Korean officials to discuss the sanctions that Secretary of State Hillary Rodham Clinton announced during her own trip to Seoul two weeks ago.
Specific new measures to be adopted by the U.S. in the next few weeks will publicly name and block the assets of entities and individuals involved in the buying or selling of arms in violation of U.N. Security Council resolutions, illegal activities to bring in hard cash or trade to divert luxury goods to Pyongyang, Einhorn said.
"We know that these activities bring hundreds of millions of dollars in hard currency annually into North Korea, which can be used to support DPRK nuclear or military programs or fund luxury goods purchases," Einhorn said at a Seoul news conference.
DPRK stands for North Korea's official name, the Democratic People's Republic of Korea.
"These measures are not directed at the North Korean people," he said. "Instead, our objective is to put an end to the DPRK's destabilizing proliferation activities, to halt illicit activities that help fund its nuclear and missile programs and to discourage further provocative actions."
He said Washington is concerned about a global network of trading firms involved in proliferation-related activities and said the U.S. would urge other nations to pressure banks in their nations to freeze those companies' accounts.
The U.S. has imposed similar sanctions on Pyongyang in the past. In 2005, Washington blacklisted the Banco Delta Asia, a bank in the Chinese territory of Macau accused of helping North Korea launder money and conduct other illicit activities.
Institutions concerned about their reputation and jeopardizing their U.S. connections stopped dealing with the bank after Washington's threat, effectively severing North Korea from the international financial system.
Einhorn said the nuclear programs of North Korea and Iran threaten global peace and security, and said Washington wants to pressure them to meet their international obligations and give up their nuclear weapons.
He praised Australia, Canada and the European Union for moving to enforce financial sanctions against Iran and North Korea, but said China's support was "critical."
China, North Korea's main benefactor and traditional ally, is a permanent, veto-wielding member of the Security Council and has balked at coming down too hard on its impoverished neighbor. China also has vast energy needs and considerable investments in Iran.
Einhorn and Daniel Glaser, the Treasury Department's deputy assistant secretary for terrorist financing and financial crimes, head to Tokyo on Tuesday and are to visit China later in the month. Other U.S. officials will travel to the Middle East and South America in coming weeks for similar meetings.
North Korea is believed to have enough weaponized plutonium for at least half a dozen nuclear bombs and last year revealed it has a uranium enrichment program that would give the regime a second way to make atomic weapons.
Five nations - China, Russia, South Korea, the U.S. and Japan - have been trying for years to negotiate with North Korea to dismantle its nuclear program in exchange for aid and other concessions. Pyongyang abandoned those talks last year.
Washington and Seoul also accuse the North of sinking a South Korean warship in March. Forty-six sailors died in an explosion that an international team of investigators say came from a torpedo fired from a North Korean submarine.
On Monday, North Korean Foreign Minister Pak Ui Chun reiterated his nation's denial of involvement in the sinking of the Cheonan warship, Indonesia's Marty Natalegawa said in Jakarta after meeting with the diplomat.
Natalegawa, his country's foreign minister, said Pak told him North Korea is ready to return to the nuclear disarmament talks.
"Indonesia welcomes DPRK's readiness for the dialogue ... and encourages efforts to revive the six-party talks process as a solution mechanism for all problems in the Korean peninsula," Natalegawa told reporters.
North Korea must first demonstrate that it is sincere about denuclearization, Einhorn said.
"We can't repeat the kind of cycle we've been through in a number of previous occasions where North Korea engages in talks, makes commitments and then abandons those talks and reneges on those commitments," he said. "We have to break those cycles, especially in the wake of the Cheonan incident."
Associated Press writers Jean H. Lee in Seoul and Irwan Firdaus in Jakarta, Indonesia, contributed to this report.