SEOUL (Reuters) - North Korea said on Saturday it would start a uranium enrichment program and weaponize all its plutonium in response to fresh U.N. sanctions, which the United States said it would work vigorously to enforce.

Pyongyang also threatened military action if Washington and its allies tried to isolate it.

The U.N. Security Council approved a resolution on Friday that banned all weapons exports from North Korea and most arms imports into the country. It authorized U.N. member states to inspect North Korean sea, air and land cargo, requiring them to seize and destroy shipped goods that violate the sanctions.

On Saturday, U.S. Secretary of State Hillary Clinton said North Korea’s “continuing provocative actions are deeply regrettable.

“They have now been denounced by everyone and have become further isolated,” Clinton said during a news conference with her Canadian counterpart in Niagara Falls, Canada.

“This was a tremendous statement on behalf of the world community that North Korea’s pursuit of nuclear weapons and the capacity to deliver those weapons through missiles is not going to be accepted by the neighbors as well as the greater international community,” Clinton said of the U.N. resolution.

KCNA news agency quoted an unnamed North Korean Foreign Ministry spokesman as saying in a statement, “We’ll take firm military action if the United States and its allies try to isolate us.

North Korea would start a program to enrich uranium for a light-water reactor, he said. Experts said North Korea lacks the technology and resources to build such a costly reactor but may use the program as cover to enrich uranium for weapons.

North Korea would “weaponize all plutonium and we’ve reprocessed more than one-third of our spent nuclear fuel rods,” the spokesman said

Clinton said the United States would work with other nations to enforce the U.N. resolution “in a vigorous way to send a clear message that we intend to do all we can to prevent continued proliferation by the North Koreans.”

Clinton said there is “still an open opportunity” for North Korea to return to six-party nuclear disarmament talks with the two Koreas, the United States, Russia, Japan and China.

SQUEEZE CONCESSIONS

North Korea has raised tensions in the region in the past months by test-firing missiles, restarting a plant to produce arms-grade plutonium and holding a May 25 nuclear test, which put it closer to having a working nuclear bomb.

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North Korea also this month sentenced two U.S. journalists, Laura Ling and Euna Lee, to 12 years hard labor for “grave crimes,” saying they illegally entered the country. Clinton had appealed for the release of the two women on humanitarian grounds.

The impoverished country for years has used its military threat to squeeze concessions out of regional powers willing to pay Pyongyang for taking steps that decrease regional risks.

In Lecce, Italy, finance ministers of the Group of Eight wealthy countries said they were “committed to the effective and timely implementation of financial measures against North Korea” as set out in the U.N. resolution.

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North Korea responded to the U.N. punishment for an April rocket launch, widely seen as a disguised long-range missile test, by saying it had restarted a plant that separates plutonium from spent fuel rods and threatening to test fire an intercontinental ballistic missile.

North Korea’s Soviet-era Yongbyon nuclear plant was being taken apart under a now-stalled disarmament-for-aid deal among Pyongyang and five regional powers. The spent fuel rods cooling at Yongbyon can produce up to one more bomb’s worth of fissile material, experts said.

The United States has long suspected North Korea, which has ample supplies of natural uranium, of having a program to enrich uranium for weapons, which would give it a second path toward producing atomic weapons.

Studies have shown that U.N. sanctions imposed on North Korea for missile testing and its only prior nuclear test in 2006 had almost no impact, while its meager trade actually increased due to lax enforcement of those measures.

“The success of financial sanctions depends heavily on how far China and the United States are willing to go to pressure North Korea,” said Jeong Hyung-gon, a research fellow at the Korea Institute for International Economic Policy.

The isolated country’s $2 billion annual trade with neighboring China, equal to about 10 percent of North Korea’s annual GDP, is its most important economic relationship. Beijing has wanted to avoid any measures that could cause North Korea’s economy to collapse and lead to chaos on its border.