SEOUL : South Korean President Lee Myung-Bak has left for a US summit likely to be dominated by North Korea's growing nuclear threat after it vowed to start a second programme to build atomic weapons.

Trade and economic issues will also figure in Lee's meeting Tuesday with President Barack Obama.

But North Korea is set to take centre-stage after the hardline communist state tested a long-range missile and a nuclear bomb and stormed out of a six-nation disarmament agreement.

On Saturday, in a defiant response to new UN sanctions, it vowed to turn all the plutonium it produces into bombs and to begin a separate weapons programme based on enriched uranium.

The North, blasting Friday's Security Council resolution as a "vile product" of a US-inspired campaign, also said any "blockade" to enforce it would be an act of war.

The 15-member Council voted unanimously to impose tougher measures to cripple the North's nuclear and ballistic missile programmes.

Resolution 1874, which does not authorise the use of force, calls for tighter inspections of cargo suspected of containing banned missile and nuclear-related items, a stricter arms embargo and new targeted financial curbs to choke off revenue for the nuclear and missile sectors.

"We are going to enforce the UN resolution," US Vice-President Joe Biden promised Sunday.

"Look, North Korea is a very destabilising element in East Asia, everyone realises that -- the Chinese realise it, the Russians realise it," he told NBC television.

"They've gone further than they've ever gone in joining us on real sanctions against North Korea. And it is important that we make sure those sanctions stick," Biden stressed.

"This is a matter of us now keeping the pressure on."

US intelligence sources quoted by American TV networks have said the North intends to respond to the resolution with a third nuclear test, following its first in 2006 and the second on May 25, which prompted the tougher sanctions.

South Korean intelligence sources quoted by Yonhap news agency said the North could have built two or three underground test sites in the northeastern district where it staged its first two tests.

But one source told the agency there are no signs yet of preparations for a third test.

The North in 2007 shut down its plants that make weapons-grade plutonium as part of a six-nation disarmament deal.

But in an angry response to the Security Council's earlier censure of its April long-range rocket launch, it vowed to restart the plants and produce more plutonium.

Until Saturday it had always denied seeking an enriched uranium programme -- a second route to an atomic bomb.

Lee will seek a written US commitment to providing a nuclear "umbrella" for Seoul in a summit joint statement, a Seoul presidential official told Yonhap.

The 28,500 US troops in the South and South Korea's 680,000-strong military went on heightened alert after the North last month renounced the armistice on the peninsula and threatened a possible attack on the South.

Several analysts and officials believe ailing leader Kim Jong-Il, 67, is intensifying military tensions to bolster his authority as he tries to put in place a succession plan involving his youngest son Jong-Un.

Biden said it was not necessarily possible to determine Pyongyang's motivations.

"We just have to deal with the reality that (North Korea) is a serious danger and a threat to the world, and particularly in East Asia," he said.

South Korean activists Monday floated 100,000 leaflets across the tense border to condemn the North's nuclear and missile developments.

Some 20 people, including defectors, released 10 huge balloons carrying bundles of leaflets across the heavily fortified frontier.

- AFP/vm