North Korea on Tuesday vowed to push ahead with its nuclear program because of what it called U.S. hostility, as an outside analysis of satellite images suggested it has ramped up work at its nuclear test site over the past month.

North Korea's statement from an unidentified Foreign Ministry spokesman came a day after a senior U.S. envoy met with high-ranking South Korean and Japanese envoys in Seoul and warned the North that an atomic test would unify the world in seeking swift, tough punishment.

North Korea made no direct threat of a nuclear test and said it was open to dialogue to resolve the nuclear standoff. An analyst, Koh Yu-hwan at Seoul's Dongguk University, said the Foreign Ministry statement was a message that "the U.S. should come to the dialogue table (with North Korea) if it wants to stop its nuclear test."

There has been widespread worry that North Korea may follow a failed April 13 long-range rocket test with a third nuclear test. Both of its previous nuclear tests, in 2006 and 2009, followed rocket launches.

Late Tuesday, Glyn Davies, the top U.S. envoy for North Korea, told reporters in Beijing, where he is meeting with Chinese counterparts to discuss the North Korean nuclear situation, that he hadn't had a chance to study the North's latest statement but that his initial sense was that it was consistent with what it has said in the past.

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 told reporters earlier 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.

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.

"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 Foreign Ministry statement said.

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Associated Press writers Hyung-jin Kim in Seoul and Alexa Olesen in Beijing contributed to this report.