'Seoul has welcomed the agreement but we believe that its faithful implementation is much more important,' said Lim Sung-nam, South Korea's lead envoy on Korean peninsula peace and security issues.

'Perhaps it is a modest first step in the right direction.'

The details of that implementation will be hammered out in Beijing on Wednesday as US human rights and aid officials meet North Korean envoys to discuss how US food shipments will work. Washington is concerned that relief goes to the country's estimated six million malnourished, not military and political elites, and that it is not sold for profit.

Washington said last week it was prepared to ship 240,000 tonnes of food over two years. Pyongyang, meanwhile, announced a moratorium on nuclear tests, uranium enrichment and long-range missile tests.

The US said Pyongyang would also allow inspectors from the International Atomic Energy Agency back into the country to visit its controversial facilities at Yongbyon.

Hammered out in talks in Beijing late last month, the deal is widely considered the most significant since protracted six-nation talks over North Korea's denuclearisation collapsed three years ago - and is raising cautious hopes that the talks could be revived once Pyongyang's young new leader, Kim Jong-un, settles into the position held by his father and grandfather.

The problem for Seoul, however, is that while it must obviously support the six-party effort, it is still waiting for apologies and/or explanations from Pyongyang for the torpedoing of its naval ship Cheonan in March 2010 and the shelling of an island community on Yeonpyeong in November that year. Forty-six sailors died in the Cheonan sinking while four South Koreans were killed on Yeonpyeong. The attacks saw tensions across the Korean demilitarised zone - the world's most heavily fortified border - escalate to their most serious point in years.

Seoul is pushing for its own talks with the North over those incidents and other matters, such as stalled aid flows - a bilateral effort that they hope will create room for the six-party framework, an effort that also involves China, the US, Japan and Russia.

A statement from Pyongyang on Friday, however, highlighted the difficulty of that effort - and its different approaches to Washington and Seoul.

Its supreme military command issued a statement renewing threats to launch a 'sacred war' against the South, and warned that slogans and banners displayed at southern barracks and facilities slandered Pyongyang's top leaders. It 'solemnly declares once again that it will indiscriminately stage its own-style sacred war to wipe out the group of traitors'.

'Those who hurt the dignity of the supreme leadership of the DPRK even a bit will find no breathing spell in this land and sky,' it said.

Several South Korean officials said they knew Pyongyang was determined to drive a wedge between Washington and Seoul, but that in the long term Seoul could not be ignored, given the lead role it would have to play in any economic rebuilding of the North. They also noted the strong personal ties between President Lee Myung-bak and his US counterpart, Barack Obama - a relationship far closer than that between any of their predecessors.

'There is no gap, no daylight' for the North to exploit, one senior official said. 'We've been closely consulted every step of the way ... and that is going to continue.'

Even if the six-party and bilateral talks resumed, it 'did not necessarily mean that we can go back to business as usual without fully addressing those [Cheonan] issues', he said.

As they watch for signs of fresh strategies from Pyongyang, South Korean officials are also warily eyeing Beijing, which they believe is continuing to deepen its links with the new leadership in Pyongyang even as North Korea courts Washington. South Korea diplomats have long bristled at a lack of detailed information about Pyongyang in discussions with Chinese counterparts.

The Foreign Ministry in Beijing welcomed news of the US-North Korea deal, and some mainland scholars note Beijing played an active role in bringing the two sides together, hoping that would revive the six-nation effort.

The two Koreas remain technically at war, their conflict having ended with a truce and fortified separation in 1953.