The United States and South Korea responded cautiously Thursday to North Korea's call for unconditional talks, saying Pyongyang must be judged on actions rather than words.

North Korea offered "unconditional" negotiations with the South Wednesday, in its most conciliatory remarks since the nuclear-armed state sent tensions on the peninsula soaring in November by shelling a South Korean island.

In an unusually cordial statement, carried by its KCNA agency, North Korea said it "courteously proposes having wide-ranging dialogue and negotiations".

Pyongyang is "ready to meet anyone, anytime, anywhere", it said, calling for "unconditional and early opening of talks" with officials with "real power and responsibility".

But South Korea dismissed the approach.

"We don't consider it as a serious offer for dialogue," Unification Ministry Spokeswoman Lee Jong-Joo told AFP.

She noted the North had regularly issued similar statements until 2007 as part of what she said was a long-standing strategy of driving a wedge between the South Korean government and its people.

The North should instead show it was serious about its obligations under a 2005 agreement on denuclearisation and apologise over the November shelling and the sinking of a South Korean warship in March last year, she said.

Washington echoed Seoul's response, saying that the North had to take "useful steps" to show that its proposal was serious.

"It needs to demonstrate it is sincere in the offer," State Department spokesman Philip Crowley told reporters, noting US Secretary of State Hillary Clinton discussed the plan with visiting Chinese Foreign Minister Yang Jiechi.

"There are still things that North Korea has to do to demonstrate a seriousness of purpose," Crowley said, such as ending its provocative behaviour and recommitting itself to a 2005 declaration for nuclear disarmament.

The offer came as the top US envoy on North Korea, Stephen Bosworth, held talks with his Chinese counterpart Wu Dawei and Vice Foreign Minister Zhang Zhijun in Beijing, after a visit to Seoul focused on reducing tensions.

Bosworth was expected to head for Tokyo later Thursday, while US Defence Secretary Robert Gates was also scheduled to meet his Chinese counterpart in a visit to Beijing starting January 9.

Foreign ministry officials said Bosworth stressed during talks in Seoul that Pyongyang needed to show it was sincere about mending ties with Seoul if six-party international talks on its nuclear programme were to resume.

The North has previously refused to discuss the nuclear issue directly with the South, saying it only wants to deal with Washington, but its latest statement suggested it may be willing to engage with Seoul on the topic.

The North's proposal put the South in a dilemma, analysts said.

"This dialogue offer places the South in a very awkward position, especially when both China and the United States want to see tension reduction through dialogue," Hong Hyun-Ik at the private Sejong Institute said.

"The North is shifting the blame for the lack of dialogue to the South."

He added that the North could conduct a third atomic test unless progress is made at stalled six-party talks on ending the North's nuclear programme -- a scenario echoed by Professor Kim Yong-Hyun of Dongguk University.

The North quit the six-nation talks in April 2009 and staged a nuclear test a month later, its second since 2006, in protest at continuing "hostile" US policy toward the communist state.

Efforts to resume the talks recently gained momentum as Beijing called for renewed dialogue and Pyongyang signalled it was willing to return to the negotiating table.

Relations between the two Koreas were stretched to breaking point after the North's shelling in November, which killed four people, including two civilians.

But tensions have softened since the New Year, with South Korean President Lee Myung-Bak offering closer economic ties if Pyongyang changes course.