North Korea vowed Tuesday to bolster its nuclear deterrent and take "self-defence" measures unless the United States halts criticism and pressure over its rocket launches and atomic programme.

A foreign ministry spokesman was hitting back at a statement Saturday from Washington and other Group of Eight nations, which condemned Pyongyang's April 13 long-range rocket launch.

The G8 said it would press for United Nations action -- an apparent reference to tougher sanctions -- in response to any further launches or to a nuclear test.

The North, in the spokesman's statement on official media, said its nuclear deterrent was developed as a response to US hostility and "we will expand and bolster it non-stop as long as this hostile policy goes on".

It said that "from the beginning" it had not envisaged "such a military measure as a nuclear test", since the aim of its launch was to put a scientific satellite into orbit for peaceful purposes.

The UN Nations Security Council strongly condemned the failed launch as breaching a ban on testing ballistic missile technology, and tightened existing sanctions.

Washington was now talking about a possible nuclear test by Pyongyang in an attempt to incite confrontation, the North's statement said.

"If the US persists in its moves to ratchet up sanctions and pressure upon us despite our peace-loving efforts, we will be left with no option but to take counter-measures for self-defence," it said, without specifying whether a nuclear test was now one of the options.

The North responded with nuclear tests after the UN imposed sanctions against its rocket launches in 2006 and 2009.

Satellite photos last month showed work under way at its atomic test site but gave no indication of whether or when a test would be staged.

The North insists that what it called a satellite launch did not breach a February deal with the United States. This promised a suspension of uranium enrichment, and a moratorium on nuclear and missile tests, in return for 240,000 tonnes of US food aid.

The US suspended plans to start food deliveries after the launch, saying it could no longer trust the North. But the North said Tuesday it would continue exercising "our sovereign right" to launch satellites for peaceful purposes.

It said a peaceful negotiated solution to the nuclear issue was still possible if the US dropped its hostile policy.

The statement said Pyongyang had notified Washington "several weeks ago" that it was still exercising restraint, even though it was no longer bound by the February accord.

"North Korea is sending a message to the United States that it must honour its earlier (February) deal," said Baek Seung-Joo of South Korea's Korea Institute for Defence Analyses.

"Today's statement prompts me to believe that there will be no nuclear test for a while as it comes under strong pressure from the international community," Baek told AFP.

"It indicates North Korea wants negotiations with the United States. However, North Korea will go ahead with a nuclear test if the US sticks to its tough line."