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, Reuters reported.
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.