"Whether North Korea tests or not, it's up to North Korea. We hope they don't do it, we call on them not to do it. It will be a mistake and a missed opportunity if they were to do it," said Glyn Davies, the U.S. special envoy on North Korea policy, when asked about the possibility of a nuclear test by North Korea.
"This is not a moment to increase tensions on the Korean Peninsula. This is the opportunity (for North Korea) to seize the moment" to engage with the outside world, Davies said.
Davies spoke to reporters in Seoul after meeting with his South Korean counterpart, Lim Sung-nam, Seoul's chief envoy to the stalled six-party talks aimed at persuading the North to give up its nuclear weapons ambition.
Before the meeting, Lim told Davies that a "close consultation between you and me, between Seoul and Washington, will be very important in this period of political transition here and beyond, and that will be our common asset in dealing with North Korea and the North Korean nuclear issue."
Asked whether South Korea and the U.S. were considering additional bilateral sanctions against North Korea, Davies replied that the allies will focus on "implementing provisions of sanctions by the U.N. Security Council, and then, we will take a look at what further steps might be necessary."
Minutes after Davies' remarks, the North's powerful National Defense Commission ratcheted up its threat of a nuclear test.
"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 North's defense commission said in a statement carried by KCNA.
It didn't say when it would detonate a nuclear device or what a "high-level" nuclear test might be, but Seoul's intelligence officials said Wednesday that North Korea has completed all technical preparations for a nuclear test and one could be carried out in a few days if the communist country makes the decision to do so.
Seoul's foreign ministry spokesman Cho Tai-young expressed deep regret at the North's threat of conducting a nuclear test.
"The government deeply regrets that North Korea made such a statement and we again strongly urge North Korea not to make any further provocations, including a nuclear test," Cho told reporters in a regular press briefing.
North Korea angrily responded to the U.N. resolution that widened sanctions in response to the North's December rocket launch, saying it will strengthen its "nuclear deterrence."
Intelligence officials said North Korea had dug a tunnel for a test at its Punggye-ri nuclear test site, but the tunnel has now been plugged with dirt and concrete, suggesting that all measuring and other equipment has already been installed inside.
North Korea had detonated nuclear devices at the Punggye-ri test site in 2006 and 2009, following long-range rocket launches.
Professor Yang Moo-jin of the University of North Korean Studies in Seoul said the Thursday statement by the North's top military body might be interpreted as a notification that it could soon conduct a third nuclear test.
"I believe that the statement by the North's National Defense Commission was a forewarning that it could conduct a nuclear test within one month," Yang said.
Some analysts have speculated that North Korea could use a uranium device for the first time after two previous tests with plutonium devices. North Korea surprised the world in late 2010 by showing its modern uranium enrichment facility to a U.S. scientist, a facility that could be easily modified to produce highly enriched uranium (HEU).
North Korea claims the uranium enrichment program is for peaceful energy development but outside experts believe that it would give the country a new source of fission material to make atomic bombs, in addition to its widely known plutonium-based nuclear weapons program.
Asked about the North's allusion to a "high-level nuclear test," an intelligence source in Seoul said, "I think it is highly likely that North Korea would conduct a third nuclear test with HEU."
Despite the North's rare threat of a nuclear test and more missile launches, the U.S. envoy Davies reminded North Korea that the Washington is "still open to authentic and credible negotiations to implement the September 19, 2005 joint statement."
"We are willing to extend our hand if Pyongyang chooses the path to peace and progress by letting go of its nuclear weapons and its multi-stage missiles," Davies said.
"If North Korea comes into compliance with Security Council resolutions and takes irreversible steps leading to denuclearization, the United States and, we believe, other partners in the six-party process will do the hard work with the DPRK (North Korea) of finding a peaceful way forward," Davies said.
"It is very much up to Pyongyang to decide," the U.S. envoy said.