SEOUL - A possible nuclear test by the Democratic People's Republic of Korea (DPRK) will be a "supremely unhelpful and retrograde step," the top US envoy said Thursday, on the same day the DPRK vowed to proceed with such tests aimed at Washington.

Glyn Davies, the US envoy on DPRK policy, arrived in Seoul Wednesday as part of his three-nation Asia tour aimed at discussing follow-up actions to expanded UN sanctions on the DPRK over its December 12 satellite launch.

The DPRK has defended its right to launch a satellite for peaceful and scientific purposes, while its critics, including the United States and the Republic of Korea, saw the launch as a disguised ballistic missile test and speculated it might be followed by a nuclear test.

"It will be a mistake and a missed opportunity if they were to do it," the envoy told reporters in reference to a possible third nuclear test by Pyongyang.

"This is not a moment to increase tensions on the Korean peninsula. This is a moment to cease the opportunity that has been out there with the new government in Seoul (and) with the renewed mandate of the president of the United States," he added.

Stressing that the 15-member UN body unanimously adopted the resolution, Davies said the move "sends a unified message" to Pyongyang that it should live up to its obligations and abandon its nuclear weapons and missiles.

"Or you will only further isolate your nation and impoverish your people," Davies said in a curt message to the DPRK, while adding that the United States is "still open to authentic and credible negotiations."

The remarks were followed by a statement by the DPRK's National Defense Commission, which vowed to proceed with the "high-level nuclear test" and continue long-rocket rocket launches.

"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," state-run KCNA news agency quoted the commission as saying.

Pyongyang conducted its first and second nuclear tests in 2006 and 2009, weeks after UN condemnations and sanctions over long-range rocket launches.

Glyn Davies, who is accompanied by Clifford Hart, the chief US envoy for six-party nuclear talks, is scheduled to make stops in China and Japan.