The UN Security Council has unanimously condemned North Korea's December rocket launch and expanded existing UN sanctions.

North Korea reacted with a vow to boost its military and nuclear capabilities.

While the resolution does not impose new sanctions on Pyongyang, diplomats said China's support for it was a significant diplomatic blow.

The resolution said the 15-member council "deplores the violations" by North Korea of its previous resolutions.

Those resolutions banned North Korea from conducting further ballistic missile and nuclear tests and from importing materials and technology for those programmes.

It also said the council "expresses its determination to take significant action in the event of a further DPRK [North Korean] launch or nuclear test".

North Korea reacted quickly, saying it would hold no more talks on the "de-nuclearisation" of the Korean peninsula and would boost its military and nuclear capabilities.

"We will take measures to boost and strengthen our defensive military power including nuclear deterrence," its Foreign Ministry said in a statement carried by state news agency KCNA.

The US special envoy on North Korea urged Pyongyang to back down from further provocative actions but left the door open for dialogue.

"If they can... begin to take concrete steps to indicate their interests in returning to diplomacy, they may find willing partners in that process," Glyn Davies told reporters as he arrived in Seoul to meet his South Korean counterparts.

Six-party talks aimed at halting North Korea's nuclear programme have involved North Korea, the United States, China, Japan, Russia and South Korea.

They have been held intermittently since 2003 but have stalled since 2008.

Russia's Foreign Minister Sergei Lavrov said North Korea should pay heed to the international community and adhere to limits on its missile and nuclear programmes.

South Korea says the North is technically ready for a third nuclear test, and satellite images show it is actively working on its nuclear site.

However, political analysts said they viewed a test as unlikely in the near-term.

There are concerns that North Korea could stage a test using highly enriched uranium for the first time.

This would give it a second path to a nuclear bomb and enable it to preserve its stocks of plutonium, which are believed to be sufficient for about 12 nuclear devices.