SEOUL – A defiant North Korea vowed Saturday to build more nuclear bombs and start enriching uranium for a new atomic weapons programme after the UN Security Council imposed sanctions for its nuclear test.

Washington's top diplomat Hillary Clinton responded by saying that the United States intends "to do all we can to prevent continued proliferation by the North Koreans."

"The North Koreans' continuing provocative actions are deeply regrettable," she told reporters at Niagara Falls on the Canadian side of the border with the US.

The North earlier described Friday's sanctions resolution as a "vile product" of a US-inspired campaign and said it would never abandon nuclear weapons and would treat any attempt to blockade it as an act of war.

The 15-member Council voted unanimously Friday to slap tougher sanctions on the North to cripple its nuclear and ballistic missile programmes.

The hardline communist state, in a foreign ministry statement reported by its official news agency, said all new plutonium it extracts would be weaponised.

One third of used fuel rods from the Yongbyon reactor have so far been reprocessed into weapons-grade plutonium, it said.

"Secondly, we will start uranium enrichment," it said in its first admission that it has such a programme -- a second route to a nuclear bomb.

In 2002, the North denied US claims that it was operating a secret uranium enrichment programme in addition to its admitted plutonium-based operation.

The plutonium-producing plants were shut down under a 2007 six-nation disarmament deal. But Pyongyang vowed to restart them after the Security Council in April condemned its long-range rocket launch.

"It has become an absolutely impossible option for the DPRK (North Korea) to even think about giving up its nuclear weapons," the statement said, adding that any attempted blockade would be considered an act of war "and met with a decisive military response."

It added: "No matter how hard the US-led hostile forces may try all sorts of isolation and blockade, the DPRK, a proud nuclear power, will not flinch from them."

Resolution 1874 passed Friday, which does not authorise the use of force, calls on UN member states to expand sanctions imposed after the North's initial nuclear test in October 2006.

It calls for tougher inspections of cargo suspected of containing banned missile- and nuclear-related items, a tighter arms embargo and new targeted financial curbs to choke off revenue for the nuclear and missile sectors.

It also "demands that the DPRK not conduct any further nuclear test or any launch using ballistic missile technology" and abandon all nuclear weapons and programmes "in a complete, verifiable and irreversible manner."

While UN chief Ban Ki-moon said the resolution sent a "clear and strong message" to North Korea, US ambassador to the UN Susan Rice said it would be no surprise if Pyongyang "reacted to this very tough sanctions regime in a fashion that would be further provocation."

US intelligence officials believe it will respond with a third atomic test, according to sources quoted by American TV networks.

Pyongyang followed up its May 25 nuclear test by launching short-range missiles, renouncing the armistice on the Korean peninsula and threatening possible attacks on South Korea.

Seoul, which has sent 600 Marine reinforcements to two border islands, denounced the North's statement as "a grave challenge" to international efforts to promote peace in the region.

"The government, together with the international community, will sternly deal with North Korea's uranium enrichment programme as well as its plutonium," a foreign ministry statement said.

The claim to have developed uranium enrichment technology is alarming, said Professor Yang Moo-Jin, of Seoul's University of North Korean Studies.

"The North has abundant natural uranium of good quality which, if combined with technology and facilities, would result in a great nuclear arsenal," he told AFP.

"This means the US policy to disarm the North by sanctions simply did not work."

Baek Seung-Joo, of the Korea Institute for Defence Analyses, said Pyongyang "will face a strong backlash from the international community for lying in the past about its uranium enrichment programme."