The most important issue concerning North Korea is the challenge it poses by continuing to pursue nuclear weapons and ballistic missiles, the top U.S. diplomat on Asia said Sunday.

Assistant Secretary of State Daniel Russel made the remarks after meeting South Korean Deputy Foreign Minister Lee Kyung-soo in Seoul, saying the talks were part of intensive and in-depth discussions between South Korea and the U.S. on important issues related to North Korea.

Russel, who accompanied Deputy Secretary of State William Burns on a trip to China and Japan, arrived in Seoul earlier in the day to brief Lee on the results of Burns' discussions in Beijing and Tokyo.

His one-day visit came amid concerns that North Korea could attempt provocations in anger over planned military exercises in the South.

Pyongyang has stepped up conciliatory gestures in recent weeks, proposing that the two Koreas halt slandering each other and demanding the South call off a set of planned military exercises with the United States. The charm offensive has spurred speculation that it might be an attempt to build justification for hostilities against Seoul.

The South has rejected the North's demand, saying it will hold the drills as planned.

Russel also told reporters that the two sides discussed relations between South Korea and Japan.

He stressed that the two nations -- as Asia's leading democracies and economies -- cannot allow bilateral tensions to continue at a time when the global economy and regional security are so important.

Frayed relations between Seoul and Tokyo have been a cause for concern for the United States as Washington seeks to build strong, three-way security cooperation with the two Asian allies in an effort to keep a rising China in check.

Seoul officials said they reiterated the need for Japan to take sincere steps toward improving Seoul-Tokyo relations and asked Washington to play a constructive role in getting Japan to do so. (Yonhap News)