The top US diplomat for East Asia told North Korea's new leaders Tuesday that they must improve relations with South Korea if they want better ties with the wider world.

Assistant Secretary of State Kurt Campbell said there is "hope in diplomacy" but stressed an unwavering military commitment to South Korea, where the United States bases 28,500 troops.

The North has stressed that its policy remains unchanged after Kim Jong-Un succeeded his father Kim Jong-Il, who died on December 17.

It vows never to deal with the South's conservative government, accusing it of failing to respect a mourning period for the late leader.

"Any engagement with North Korea, if North Korea wants a better relationship with the world, the first stop is a better relationship with South Korea," Campbell told a forum organised by the Korea Society in Seoul.

Military tensions have been high since the South accused the North of responsibility for two border incidents in 2010 which killed 50 South Koreans.

Some analysts believe the untested new young leader may stage a border provocation to bolster his credentials with the North's powerful military.

Campbell, quoted by Yonhap news agency, said he was confident of deterring North Korea because of the US partnership with the South.

"Our partnership is so tight, our consultations are so close that I'm confident that whatever happens, we will be able to meet the challenges together," he said.

Diplomatic efforts have been underway for months to restart long-stalled six-nation talks on the North's nuclear disarmament.

"There is hope in diplomacy," said Campbell, who will meet the South's nuclear negotiator Lim Sung-Nam on Wednesday.

"That hope in diplomacy rests in the reality of a very strong deterrence and military commitment that will be unwavering."

Lim will also visit Russia next week for talks with Deputy Foreign Minister Igor Morgulov.

The talks grouping the two Koreas, China, Japan, Russia and the United States have not been held since December 2008. Pyongyang walked out in April 2009 and tested its second atomic weapon a month later.

The US and North Korea last year held two rounds of talks aimed at restarting the six-party negotiations. A third round was reportedly scheduled in Beijing but the death of Kim Jong-Il put the process on hold.

Last week, Russian Foreign Minister Sergei Lavrov told Japan's NHK television that six-party talks could resume in the first half of this year.

Washington's special representative for North Korea, Glyn Davies, was due to arrive in Moscow on Tuesday. He is accompanied by the US envoy to the nuclear talks, Clifford Hart.

The North wants the six-party forum to resume unconditionally. The United States and its allies say it must show it is serious by shutting down a newly disclosed uranium enrichment plant.