North Korea's threat to "weaponise" its plutonium stocks is "provocative" and "deeply regrettable", US Secretary of State Hillary Clinton says.
Mrs Clinton said Washington would vigorously impose new UN sanctions against Pyongyang.
"The North Koreans' continuing provocative actions are deeply regrettable. They have now been denounced by everyone and have become further isolated," she said.
Pyongyang said it would preprocess more plutonium, start enriching uranium and weaponise its existing stockpile in response to new UN sanctions agreed to on Friday.
North Korea said it would view any US-led attempts to "blockade" it as "an act of war".
"The resolution represented a unified response to the provocative actions that have been taken by the North Koreans over the last several months," Mrs Clinton added.
"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," she said.
The Security Council's latest resolution, passed unanimously in response to a nuclear bomb test in May, bans all weapons exports from North Korea and most arms imports.
UN member states are authorised to inspect North Korean sea, air and land cargo, and must seize and destroy any goods transported in violation of the sanctions.
China and Russia come to the party this time
Both China and Russia, which in the past have been reluctant to support punitive measures against North Korea, backed the US-drafted resolution, which is now binding under international law.
Whether China abides by it remains to be seen. Beijing ignored an earlier round of sanctions against Pyongyang passed after North Korea's first nuclear test in October 2006.
The resolution also urges North Korea to return to the six-nation aid-for-disarmament talks with the United States, China, Russia, Japan and South Korea and offers the possibility of suspending the sanctions if North Korea changes course.