SEOUL — North Korea appears to be readying for a possible third nuclear test as early as next March, a newspaper reported Wednesday, as New Mexico Gov. Bill Richardson headed to Pyongyang on an unofficial diplomatic foray.
U.S. and South Korean intelligence have been watching the North's nuclear sites for any activity.
Analysts say the North could use a test to try to gain leverage in international talks it is seeking and secure aid to prop up its destitute economy.
The New York Times reported Wednesday that the Obama administration has concluded that that North Korea's new plant to enrich nuclear fuel uses technology that is "significantly more advanced" than what Iran has assembled.
North Korea already has the fuel for as many as 12 nuclear weapons, the newspaper said.
South Korea's Chosun Ilbo daily on Wednesday cited an intelligence official from Seoul as saying a tunnel was being dug at the country's nuclear test site that could be completed in March next year, possibly heralding a new nuclear test.
South Korea's foreign ministry said it was closely monitoring the site and said there was no concrete evidence to show the North Koreans were preparing for a third test.
The amount of earth removed from the site in Punggye township, in a northeastern region of North Korea, indicated the tunnel was about 550 yards deep, half the depth needed for a nuclear test, the Chosun Ilbo report said.
"North Korea is digging the ground pretty hard ... at its two major nuclear facilities," a South Korean intelligence official was quoted as saying.Slideshow: Daily life in North Korea (on this page)
"At this rate, (the Punggye tunnel) will reach (the) 1 km that is needed for a nuclear test by March to May," a separate intelligence source told the newspaper.
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North Korea is also speeding up work on new construction at its main Yongbyon nuclear complex, where it revealed a previously unknown uranium enrichment facility last month, the newspaper quoted intelligence sources as saying.
South Korea's foreign ministry declined to confirm the details of the report, but said: "Nothing has been confirmed that would prove the North is preparing to conduct a nuclear test."
The South's nuclear envoy, Wi Sung-lac, was in Moscow meeting with his Russian counterpart, in the same week that Russian Foreign Minister Sergei Lavrov met North Korean Foreign Minister Pak Ui-chun and chided him over the North's nuclear program.
Civil defense drill
The report coincided on Wednesday with South Korea's largest civil defense drill in recent years after North Korea shelled an island near their disputed maritime border, killing four people, last month.
The exercise on a busy week day brought traffic to a standstill nationwide and saw mass evacuations to bomb shelters.
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Prime Minister Kim Hwang-shik said the aim was to heighten readiness for a possible North Korean air raid and he also warned the North to expect reprisals if there was another attack.
"The government is ready to demonstrate that there will be due price to pay for any future aggression," he said.
Analysts say North Korea's unveiling of a modern uranium enrichment facility and preparations for another nuclear test were likely to be ploys to pull regional powers back to the negotiating table, where Pyongyang hopes to secure aid.
The impoverished state has in the past won economic assistance and diplomatic attention at six-country talks aimed at persuading it to abandon its nuclear ambitions.Slideshow: The life of Kim Jong ll (on this page)
Deputy U.S. Secretary of State James Steinberg was in Beijing to consult with the Chinese on North Korea and Richardson was on his way to the North.
"My objective is to try to get North Korea to calm down a bit, see if we can reduce tension in the Korean peninsula," Richardson said as he arrived at an Albuquerque airport for the first leg of a commercial flight to Beijing and then to North Korea.
The governor will return to New Mexico on Dec. 20.
This will be Richardson's third trip to North Korea since he became governor in 2003.
Richardson served as U.N. ambassador in the Clinton administration and often has played the role of an unofficial diplomatic troubleshooter, including missions to secure the release of hostages in Sudan, Iraq and North Korea.
The State Department has said Richardson isn't delivering a message to North Korea for the U.S. government. However, Richardson's contacts with North Korean officials may provide insights for diplomats trying to persuade North Korea to abandon its nuclear weapons program.
North Korea showed a uranium enrichment facility, which could give it a second route to make nuclear bombs, at the Yongbyon site to a U.S. expert in November and later announced it was operating such a program under a "peaceful" energy project.Video: Photos show possible advances in N. Korea nuclear program (on this page)
South Korea's foreign minister said on Tuesday he suspected there were more facilities in addition to Yongbyon where the North was enriching uranium. A media report said Pyongyang had three to four such plants.
North Korea conducted nuclear tests at the Punggye site in 2006 and 2009, when detonations in tunnels were detected by U.S. and South Korean monitoring.
The U.N. Security Council condemned last year's test and imposed tough sanctions aimed at banning North Korea's arms trade and cutting off funding for such programs.
Analysts say ailing leader Kim Jong-il's plan to transfer power to his son Jong-un is also creating domestic political pressure, as the regime resorts to military grandstanding to try to build legitimacy for the untested and previously unknown successor.
The Associated Press, Reuters and msnbc.com staff contributed to this report.