Two senior U.S. diplomats met South Korean officials Tuesday to prepare for nuclear disarmament talks with North Korea and discuss enforcing sanctions against the communist regime for its atomic weapons test.

"Let me confess that many challenges are ahead of us. We need confidence in our alliance," South Korean Vice Foreign Minister Yu Myung-hwan said at the start of a meeting with Nicholas Burns, U.S. undersecretary of state for political affairs.

"Our relationship is very important to the United States," Burns said earlier.

Further details of the discussion weren't immediately available, but the officials were expected to discuss how to make progress when a fresh round of six-nation talks on North Korea's nuclear program resumes.

North Korea agreed last week to end its yearlong boycott of the negotiations with China, Japan, South Korea, Russia and the U.S. in the first sign of a relaxation of tensions after Pyongyang's Oct. 9 nuclear test. A resumption date for the talks has yet to be set.

Robert Joseph, U.S. undersecretary of state for arms control and international security, who is traveling together with Burns, also held separate talks with Park In-kook, a deputy foreign minister.

Joseph said afterward that the meeting focused on the "importance of full implementation" of a U.N. sanctions resolution adopted to punish North Korea for its first-ever nuclear test.

"We've had very useful, very productive discussions," he said, declining to give further details.

South Korea's Foreign Ministry said Joseph wasn't originally planning to come to Seoul, but his itinerary was changed at the last minute. Local media have interpreted that as an indication that Washington wants to pressure Seoul to vigorously implement U.N. sanctions on the North.

South Korea has been struggling to strike a delicate balance between its obligations to punish the North under the U.N. sanctions resolution and concerns that aggravating its volatile neighbor could destabilize the region. South Korea has been noncommittal to a U.S. request to expand its role in a U.S.-led international program aimed at interdicting shipments of weapons of mass destruction.

North Korea is a primary target of the U.S.-led Proliferation Security Initiative. Seoul, which has participated in the program only as an observer, is concerned stopping and searching North Korean ships may lead to clashes with the North.

South Korean President Roh Moo-hyun said Monday that efforts to get rid of North Korea's nuclear program shouldn't lead to a fresh conflict on the divide peninsula.

In Tokyo, the U.S. diplomats agreed with Japanese officials to seek a meeting of all five dialogue partners of North Korea on the sidelines of this month's summit of the Asia-Pacific Economic Cooperation forum in Vietnam.

The two sides also called for concrete results from future six-party talks on their demands that North Korea give up its quest for a nuclear weapon and allow outside verification that it is complying with such a pledge.

The U.S. officials were to head to Beijing later Tuesday for meetings with Chinese and Russian officials.