The flurry of diplomacy coincided with a report that North Korea might be about to ratchet up tensions by upgrading a launch site used to test missiles that can hit all of South Korea and most of Japan.
U.S. Assistant Secretary of State Christopher Hill was scheduled to have a second day of talks with the North's top nuclear envoy in the communist state's capital on Thursday, the State Department said.
He was expected to drive back to Seoul afterwards to brief officials on his visit, a State Department official said.
A South Korean Foreign Ministry spokesman said that Japan's nuclear envoy was expected in Seoul on Thursday and that Hill would go to Beijing on Friday.
China, along with Japan, South Korea, Russia and the United States, is trying to push North Korea into giving up nuclear weapons.
Local media reported that Hill was ready to offer a compromise on how to check statements by the North about its nuclear program in an effort to revive a faltering disarmament-for-aid deal agreed with regional powers.
But State Department spokesman Sean McCormack said on Wednesday that Hill was not going to Pyongyang with an offer to change the "substance" of any verification mechanism.
The North baulked at U.S. verification demands, fearing that they would be too intrusive. Washington countered by saying it would only remove Pyongyang from its terrorism blacklist once the North had agreed to a "robust" verification system.
LOOKS TO BUSH SUCCESSOR?
Analysts have said North Korea might be trying to pressure the outgoing Bush administration as it looks for diplomatic successes to bolster its legacy. The North might also think it could reach a better deal under a new U.S. president.
The International Atomic Energy Agency said last week that North Korea had expelled U.N. monitors from the plutonium-producing part of its Yongbyon nuclear plant and planned to start reactivating the Soviet-era reactor in a week's time. Monitors remained in other areas of the complex, it said.
North and South Korea held their first discussions on Thursday since Pyongyang cut off inter-Korean dialogue early this year in anger at the policies of the South's conservative new president whp wanted to tie aid to disarmament.
The North's proposal for working-level military talks was seen by analysts as a possible olive branch heralding future discussions. But Thursday's session ended shortly after it began with no indication that any progress had been made.
North Korea has also been upgrading facilities at the site of its past missile tests in what might be preparations for another long-range missile launch, a Seoul daily reported.
"North Korea has replaced a tower crane that propped up the launch pad and is improving the stability of missile fittings such as reinforcing missile supporters," the Dong-a Ilbo quoted a South Korean government source as saying.