The United States is open to diplomacy with North Korea's new leaders but they must improve frosty ties with South Korea and show seriousness about nuclear disarmament, a senior US diplomat said Wednesday.

Assistant Secretary of State Kurt Campbell said the North must take "necessary steps" before any revival of long-stalled six-party nuclear disarmament talks.

"We are open to diplomacy with North Korea but there are a very clear set of steps necessary," he said.

The US and its allies Japan and South Korea "are still waiting to see whether the new government in North Korea is prepared to take the necessary steps", Campbell told reporters.

He did not elaborate. The US and its allies have repeatedly called on the North to shut down its uranium enrichment plant -- which experts say could be reconfigured to make weapons -- before the nuclear negotiations can resume.

Campbell, speaking after talks with South Korea's nuclear envoy Lim Sung-Nam, also stressed that Pyongyang must make peace with Seoul if it wants a better relationship with Washington.

He said he and Lim "underscored again very clearly that the road to these improved relations runs through Seoul for North Korea".

The North has stressed that its policy remains unchanged after the death of longtime leader Kim Jong-Il on December 17, and the takeover by his youngest son Jong-Un.

It has vowed never to deal with "traitors" in the South's conservative government, accusing them of failing to respect a mourning period for the late leader.

China, the North's economic prop and sole major ally, moved quickly to give its backing to the young and untested Jong-Un after his father's sudden death.

It also ordered swift deliveries of rice and fuel to the impoverished state in a gesture of support for the son, according to a Japanese media report.

Campbell urged China to share its thoughts on the new leadership: "We want them to share with us more their perspectives and their plans."

The nuclear talks grouping the two Koreas, China, Japan, Russia and the United States were last held in December 2008. Pyongyang walked out in April 2009 and tested its second atomic weapon a month later.

The US and North Korea met twice last year to try to restart the negotiations. A third round was reportedly scheduled in Beijing but Kim's death put the process on hold.

The North wants the six-party talks, which have dragged on since 2003, to resume unconditionally.

The United States and its allies say the North must show it is serious about scrapping its nuclear operations, notably by shutting the enrichment plant.

South Korea's top official for cross-border ties told a forum that Kim's death opened a window for change in the communist North.

"Chairman Kim Jong-Il's death heightened uncertainties," Unification Minister Yu Woo-Ik said in a speech.

"North Korea also faces opportunities for changes. I urge the North to drop its old attitude and take the road for reconciliation, exchange and cooperation," Yu said.

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

Yu said the North's past provocations had ostracised it from the rest of the world, but he called for cross-border dialogue on all pending issues.

While its economy had missed out on industrialisation and globalisation, "if the North decides on improving people's livelihood, the South is ready to lend a helping hand", the minister said.