The United States and its negotiating partners are preparing a "comprehensive package" of incentives to encourage North Korea to scrap its nuclear weapons, a senior US envoy said Monday.

Kurt Campbell, assistant secretary of state for East Asia and Pacific affairs, did not elaborate on the package which is still being finalised.

But he said it would require Pyongyang to take "irreversible" steps towards denuclearisation in return.

Yonhap news agency said he made his comments at a breakfast meeting with senior South Korean journalists. A US embassy spokesman confirmed the main points of the Yonhap report.

Six-nation nuclear disarmament talks grouping the two Koreas, China, Japan, the US and Russia began almost six years ago but bogged down last December.

After the United Nations Security Council censured its April 5 long-range rocket launch, the North announced it was quitting the talks and restarting its atomic weapons programme.

It staged its second nuclear test on May 25, prompting the Council to adopt a resolution imposing tougher sanctions.

South Korea's coast guard said it was drawing up a plan on how to inspect North Korean ships suspected of carrying banned weapons items.

"It is a follow-up to Seoul's joining the Proliferation Security Initiative," a coast guard spokesman said, referring to a US-led campaign launched in 2003 to stop ships suspected of carrying weapons of mass destruction.

The North has said it sees Seoul's decision to join the initiative as a declaration of war.

It was unclear whether Seoul would try to stop North Korean ships in South Korean waters, and if so whether the navy or the coast guard would be used.

Campbell, on arrival from Japan Saturday, had outlined a "two-track strategy" involving tough enforcement of sanctions but also negotiations -- if the North is willing to give up its nuclear ambitions.

He said at that time the comprehensive package would be "attractive" to North Korea if it returns to the talks and takes irreversible steps to disarm.

Campbell, making his first trip to South Korea since taking over the post last month, met Seoul's nuclear negotiator Wi Sung-Lac Monday morning.

"We have to closely work together on how to implement the Security Council resolution and on the other hand... we have to think about resumption of dialogue as well," Wi told the US envoy.

Last Thursday the Council tightened sanctions further, imposing a travel ban on five North Korean officials and asset freezes on five more entities involved in the missile or nuclear programmes.

After meeting Foreign Minister Yu Myung-Hwan Monday afternoon, Campbell was to leave for Thailand for the ASEAN Regional Forum on security issues. North Korea is expected to be high on the agenda.