SEOUL, May 22: North Korea has ramped up work at its nuclear test site over the last month, according to an analysis of satellite images released on Tuesday, a day after the top US envoy for North Korea warned Pyongyang that an atomic test would unify the world in seeking swift, tough punishment.

Glyn Davies' comments after meetings Monday in Seoul with his South Korean and Japanese counterparts reflect widespread worry that North Korea may follow a failed April 13 long-range rocket test with its third nuclear test. Both of its previous nuclear tests, in 2006 and 2009, followed rocket launches.

Satellite images taken by DigitalGlobe and GeoEye in the past month show heightened activity at the Punggye-ri nuclear test site in North Korea's northeast, including mining carts, excavation equipment and a large amount of debris taken from inside a tunnel and piled around its entrance, James Hardy, IHS Jane's Asia Pacific specialist, said in a statement Tuesday. The most recent image was from May 9.

South Korean intelligence 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. A new tunnel is likely needed because existing ones probably caved in and became contaminated with radioactive material after previous tests.

North Korea is being led by Kim Jong Un, who took power in December following the death of his father, Kim Jong Il. He has since vowed to place top priority on his impoverished country's military.

“We are obviously in a bit of an uncertain period with North Korea” after the rocket launch, Davies told reporters at the Foreign Ministry.

“It is very important that North Korea not miscalculate again and engage in any future provocation.”

Describing Washington's policy on North Korea as “engagement on the one hand, pressure on the other,” Davies said that the “engagement aspect remains open. If they make the right choices, there can be a different future for North Korea.”Another nuclear test, however, would result in “swift and sure” punishment at the UN Security Council, he said.

Davies indicated US frustration with North Korea's announcement of its planned rocket launch just two weeks after the countries had struck a food aid-for-nuclear freeze deal — the result of months of tedious, back-and-forth negotiations and seen as something of a breakthrough at the time.

The decision sent a “signal that they can't be trusted to follow through on their own undertakings and their own promises,” Davies said.

“Words are no longer, quite frankly, interesting to us. What we want to see is actions from North Korea,” Davies said.

Washington and other nations called the April rocket launch a cover for a test of missile technology that could be used to attack the United States — and therefore a violation of the US-North Korea deal. North Korea said the rocket, which broke into pieces over the Yellow Sea shortly after liftoff, was meant to send an observational satellite into orbit.—AP