SEOUL, South Korea >> North Korea has ramped up work at its nuclear test site, according to an analysis of satellite images released Tuesday, a day after a senior U.S. envoy warned the North 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.

North Korea, meanwhile, shot back in a statement Tuesday, saying it will keep developing its nuclear program if the United States continues to "stifle" the country.

A Foreign Ministry spokesman made no direct threat of a nuclear test, but said North Korea feels compelled to strengthen its "nuclear deterrent" in the face of U.S. hostility. However, the spokesman also said North Korea is open to dialogue to resolve the standoff.

Koh Yu-hwan, a North Korea professor at Seoul’s Dongguk University, said the North’s message is that "the U.S. should come to the dialogue table (with North Korea) if it wants to stop its nuclear test."

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 Korean leader Kim Jong Un, who took power in December following the death of his father, Kim Jong Il, has vowed to place top priority on his impoverished country’s military.

"It is very important that North Korea not miscalculate again and engage in any future provocation," Davies, the top U.S. envoy for North Korea, told reporters at the South Korean Foreign Ministry. "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 U.N. Security Council, he said.

North Korea announced its planned rocket launch just two weeks after it had struck a food aid-for-nuclear freeze deal with Washington — the result of months of tedious, back-and-forth negotiations that was 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 U.S.-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.

The North Korean Foreign Ministry comments were in response to an earlier statement by the Group of Eight that warned the North that it faces more sanctions if it continues to threaten the stability of the region with provocative acts such as rocket launches.

"If the U.S. persists in its moves to ratchet up sanctions and pressure upon us despite our peace-loving efforts, we will be left with no option but to take countermeasures for self-defense," the North’s statement said.

___

Associated Press writer Hyung-jin Kim contributed to this report.