Beijing urged restraint on Thursday after Pyongyang said it is planning a third "higher level" nuclear test and more rocket launches aimed at the US.

Washington said the nuclear test, if staged, would be "a mistake and a missed opportunity" for Pyongyang.

Experts said the planned nuclear test is likely to use highly enriched uranium instead of the plutonium used in Pyongyang's previous tests. The test date will probably be Feb 16, the birthday of Kim Jong-il, or April 15, the birthday of Kim Il-sung.

"All parties should refrain from action that might escalate the situation in the region," Foreign Ministry spokesman Hong Lei told reporters in Beijing.

China called for the resumption of the Six-Party Talks, which have been stalled since 2009.

"Currently, the situation on the Korean Peninsula is very complicated and sensitive," Hong said.

The comments came several hours after Pyongyang's top military body, the National Defense Commission, announced a third nuclear test would be conducted, without giving a date.

"We do not hide that a variety of satellites and long-range rockets that will be launched by the DPRK, one after the other, and a nuclear test of a higher level that will be carried out by it in the upcoming all-out action - a new phase of the anti-US struggle that has lasted century after century - are targeted against the US, the sworn enemy of the Korean people," the commission said in a statement, according to state news agency KCNA.

The statement was the latest response from Pyongyang to a United Nations Security Council resolution on Tuesday condemning the December rocket launch by the Democratic People's Republic of Korea. The resolution also expanded existing UN sanctions.

Pyongyang's previous nuclear tests in 2006 and 2009 were carried out within months of long-range rocket launches.

Seoul-based Yonhap News Agency, citing an ROK intelligence source, reported earlier that Pyongyang had finished technical preparations and could conduct a test within days.

A US think tank said last month that, based on satellite photos, the DPRK had repaired damage at its nuclear test site and could conduct a detonation within weeks.

The DPRK statement said that "settling accounts with the US needs to be done with force, not with words, as it regards jungle law as the rule of its survival".

The US has more than 28,000 troops in the Republic of Korea.

Pyongyang has cited the US troops as a key reason to develop nuclear weapons.

In an earlier response, the DPRK rejected involvement in any future disarmament negotiations. Washington urged Pyongyang on Thursday not to proceed with the nuclear test.

"Whether North Korea tests or not is up to North Korea," Glyn Davies, the top US envoy for DPRK diplomacy, said in Seoul. "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," Davies said after a meeting with ROK officials, speaking shortly before Pyongyang's statement. Davies will soon fly to Beijing.

"This is not a moment to increase tensions on the Korean Peninsula," he said.

Seoul on the same day voiced deep regret at the plan. "Our government once again strongly urges North Korea to pay heed to the international community's constant warnings and not commit any further provocative acts including nuclear tests," ROK Foreign Ministry spokesman Cho Tai-young said.

The UN Security Council resolution threatened "significant action" if Pyongyang fulfilled a third nuclear test.

It is widely speculated that the nuclear test of a "higher level", as Pyongyang announced, would use highly enriched uranium instead of plutonium. The DPRK declared in 2009 that it would begin enriching uranium.

Zhang Liangui, a professor on Korean Peninsula studies at the Party School of the Communist Party of China Central Committee, said it's still early for the DPRK to conduct a successful nuclear test. "Usually a successful nuclear test comes after about 10 trials," he said.

And the DPRK has an even longer way to go before it can potentially send nuclear warheads to the US, he added.

As for the time of the test, Zhang said it will likely be on April 15 - Kim Il-sung's 101st birthday. Reuters quoted analysts as saying that the test could also be staged as early as Feb 25 as the ROK's President-elect Park Geun-hye is sworn in.

But Liu Jiangyong, vice-dean of the Institute of Modern International Relations at Tsinghua University, said it is too early to say whether and when the DPRK will conduct nuclear test as it is now "very emotional" after the UN resolution.

Though Washington and Seoul are reportedly discussing the possibility of bilateral sanctions against Pyongyang, aside from the UN resolution, Liu said that will not really work as trade ties are quite weak.

Pyongyang's reaction now is the result of sanctions, and sanctions alone cannot solve the nuclear impasse, he said.

Yu Shaohua, director of the Department for Asia-Pacific Security and Cooperation Studies of the China Institute of International Studies, noted that "Pyongyang's pursuit of a nuclear deterrence is due to deep security concerns. In that regard, relevant parties, including the US and the ROK, are responsible for the situation."

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