A team of U.S. officials visited the South Korean capital to confer with about further sanctions and other measures against North Korea.
The United States is preparing to roll out new sanctions to squeeze North Korea, which is believed to earn hundreds of millions of desperately needed dollars from illicit activities for its cash-strapped economy.
The U.S. Treasury Department's top official on terrorist financing and financial crimes says the new sanctions - to be unveiled in several weeks - will target North Korea's counterfeiting of U.S. currency, its narcotics trafficking and other illicit activities.
The aim, U.S. officials say, is to push Pyongyang to give up those activities, as well as its nuclear weapons programs.
Daniel Glaser said Monday in Seoul that naming the companies involved in crimes will have a chilling effect on their operations.
"Once it is known throughout the world that a particular entity is involved in illicit activities for North Korea - or illicit activities of any kind, for that matter - it becomes increasingly difficult for that entity to gain access to the international financial system," he said.
The American officials, who head to Japan on Tuesday, will also soon visit China to encourage cooperation in stopping nuclear proliferation by North Korea and Iran.
Robert Einhorn, the U.S. State Department's special advisor for non-proliferation and arms control, acknowledges that some countries fear that if they stop doing business with Pyongyang and Tehran, Chinese companies will move in to fill the void.
"We want China to be a responsible stakeholder in the international system," said Einhorn. "And that means cooperating with U.N. Security Council resolutions. And it means not back-filling, not taking advantage of the responsible self-restraint of other countries."
China is North Korea's only significant ally and its economic lifeline. China, with a large and growing appetite for energy, also is a major investor in Iran's oil and gas sector.
American officials also want South Korea to implement strong new measures against Iran. Glaser terms it "absolutely vital" that Seoul be a full partner in targeting Iranian entities involved in its nuclear activities.
Einhorn says Monday's discussions here included the steps South Korea can take to send a message to Iran.
Einhorn virtually rules out any chance of a quick return to the stalled six-nation talks on ending North Korea's nuclear programs. Pyongyang had agreed to give up the programs in return for economic aid and greater diplomatic recognition.
Einhorn notes a pattern of Pyongyang walking out of the talks and reneging on its commitments, saying "we have to break that cycle."
"The North Koreans have to demonstrate that they're serious about abiding by their commitments," he added. "Unless they do, then talks for talks' sake are not interesting to us."
Tension has escalated on the Korean peninsula in recent months. The U.S., South Korea and several other countries blame Pyongyang for sinking a South Korean navy ship in March. North Korea denies attacking the ship.