An analysis of satellite images released Tuesday shows North Korea has ramped up work at its nuclear test site over the last month.
The news comes a day after the top US envoy for North Korea, Glyn Davies, warned Pyongyang that an atomic test would result in "swift and sure" punishment, the Associated Press reported.
International concern has risen that North Korea may follow a failed April 13 long-range rocket test with its third nuclear test — a pattern that happened in 2006 and 2009 — according to the LA Times.
The Unha-3 rocket flew for less than two minutes before breaking up over the Yellow Sea west of South Korea.
The US believes the rocket was a test of long-range missile-firing capability.
Meanwhile, satellite images taken by DigitalGlobe and GeoEye over the past month show heightened activity at the Punggye-ri nuclear test site in North Korea's northeast, the AP reported in a separate article Tuesday.
South Korean officials said last month that satellite images showed North Korea was digging a new tunnel in what appeared to be preparation for another nuclear test at the site.
Existing tunnels probably caved in and became contaminated with radioactive material after previous tests, the intelligence officers reportedly said.
The US-Korea Institute at Johns Hopkins School of Advanced International Studies said the satellite images of North Korea's Tonghae Satellite Launching Ground "strongly suggest that this new pad is designed to launch rockets larger than the recently tested Unha, either more capable, liquid-fueled space launch vehicles or missiles with intercontinental ranges," according to Reuters.
According to Reuters, experts say the reclusive country already has enough fissile material for at least six nuclear bombs.
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Davies, meantime, said Monday: "It is very important that North Korea not miscalculate again and engage in any future provocation."
Davies described it as "an uncertain period with North Korea" after the rocket launch and the leadership handover to Kim Jong Un, the young son of Kim Jong Il, who has vowed to place top priority on North Korea's military.
Davies' comments came after he met in Seoul with representatives from the other nations of the stalled six-party talks on North Korea's nuclear programs.
He plans to travel to China and Japan later this week.
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