North Korea said Thursday it planned to carry out a third nuclear test and more rocket launches aimed at its "arch-enemy" the United States in response to tightened UN sanctions, but offered no timeframe.

"We do not hide that the various satellites and long-range rockets we will continue to launch, as well as the high-level nuclear test we will proceed with, are aimed at our arch-enemy the United States," the National Defence Commission said.

"Settling accounts with the US needs to be done with force, not with words," it added.

The mention of the test came towards the end of a commission statement carried by the official Korean Central News Agency.

It did not specify when the test might be carried out, saying only that it would be part of an "upcoming all-out action" that would mark a "new phase" of the country's anti-US struggle.

It also did not elaborate on the meaning of "high-level". Some experts have predicted that the North's next atomic test might be of a uranium bomb, rather than the plutonium devices it detonated in 2006 and 2009.

Such a development would indicate that North Korea had mastered the sophisticated technology needed to produce highly enriched uranium (HEU).

"The statement reads like typical North Korean brinkmanship, and we can't definitely say a test is imminent," said Kim Yong-Hyun, professor of North Korea studies at Dongguk University.

"But it's highly possible that it will use HEU for the test when it happens," Kim said.

Much of the statement was devoted to condemning Tuesday's announcement by the UN Security Council of expanded sanctions against Pyongyang in response to its long-range rocket launch last month.

"We absolutely refute all the illegal and outlawed resolutions adopted by the Security Council," the commission said.

Tuesday's resolution, proposed by the United States, was adopted unanimously by the 15-nation council, including the North's sole major ally China.

As well as adding a number of North Korea entities and individuals to an existing UN sanctions list, the resolution threatened "significant action" if the North stages a nuclear test.

North Korea's foreign ministry reacted defiantly on Wednesday, when it also gave the first hint that Pyongyang would react with a nuclear test by vowing "physical actions" to boost its nuclear deterrent.

South Korea's Yonhap news agency, citing a South Korean intelligence source, reported that Pyongyang had finished technical preparations and could conduct a test within days of a decision by leader Kim Jong-Un.

Last month, a US think-tank reached a similar conclusion based on satellite photos, suggesting the North had repaired rain damage at its nuclear test site and could conduct a detonation at two weeks' notice.

China, which had sought to shield North Korea from harsher sanctions sought by the United States and its allies, appealed on Wednesday for restraint and diplomatic efforts to avoid a dangerous escalation of tensions.

"The DPRK's (North Korea's) satellite launch as well as speculation of a nuclear test highlight the urgency and importance of settling relevant issues on the Korean peninsula," foreign ministry spokesman Hong Lei told reporters.

The defence commission's statement coincided with a visit to Seoul by the US special envoy on North Korea, Glyn Davies, who urged Pyongyang not to challenge the international community with a third test.

"Whether North Korea tests or not, it's up to North Korea," Davies told reporters after a meeting with his South Korean counterpart, speaking shortly before the North's statement.

"We hope they don't do it, we call on them not to do it. It would be a mistake and a missed opportunity if they were to do it," he said.