The United States on Saturday denounced North Korea's launch of a long-range rocket as "destabilizing and provocative."

The unpredictable hermit state made good on its threat to launch a satellite-bearing rocket, an operation widely seen as a covert ballistic missile test despite Pyongyang's insistence that it is part of a purely scientific program.

A US defense official said the launch vehicle "appears to have reached space."

Pyongyang had already detonated what it claimed was a powerful thermonuclear bomb on January 6.

The rocket launch "represents yet another destabilizing and provocative action and is a flagrant violation of multiple United Nations Security Council resolutions," White House National Security Advisor Susan Rice said in a statement.

"North Korea's missile and nuclear weapons programs represent serious threats to our interests -- including the security of some of our closest allies -- and undermine peace and security in the broader region."

On Friday, the US and South Korean presidents spoke with their Chinese counterpart in separate telephone calls to demand punitive measures against Beijing's close ally.

"We will continue to work with our partners and members of the UN Security Council on significant measures to hold the DPRK to account," US Secretary of State John Kerry said.

"Now is the time to do so in a firm and united way, with measures that make clear the determination of the international community to address the pursuit of nuclear and ballistic missile capabilities by the DPRK and this most recent destabilizing and unacceptable challenge to our common peace and security."

- 'All necessary steps' -

The White House and its allies want to respond with a UN resolution that would slap more sanctions on the North.

But they must first win the backing of UN veto power China, which has in the past shielded its neighbor.

"We condemn today's launch and North Korea's determination to prioritize its missile and nuclear weapons programs over the well-being of its people, whose struggles only intensify with North Korea's diversion of scarce resources to such destabilizing activities," Rice said.

"The United States is fully committed to the security of our allies in the region, and we will take all necessary steps to defend ourselves and our allies and respond to North Korean provocations."

She also urged the international community to "stand together and demonstrate to North Korea that its reckless actions must have serious consequences."

US Strategic Command said the missile launch took place at 0029 GMT Sunday and had a southerly trajectory over the Yellow Sea.

"NORAD determined that at no time was the missile a threat to North America," it added, referring to North American Aerospace Defense Command.

North Korea is already subject to numerous UN sanctions over previous nuclear and rocket tests, but South Korean President Park Geun-Hye said on Thursday its continued provocative behavior showed these had been ineffective.

The only solution, she argued, was to impose sanctions harsh enough "to make it realize that it will not survive unless it gives up its nuclear program."