May 24, 2012 -- Updated 1023 GMT (1823 HKT) The U.S. has warned Pyongyang that such a test would be "a serious miscalculation" Many analysts assume an atomic test by North Korea is just a matter of time following the failure of a controversial rocket launch last month. A satellite image from mid-April cited by Janes shows a full mining train, including an engine and several carts, outside of the tunnel at the nuclear site. North Korea said Tuesday that it would press on with its nuclear program as a response to what it described as hostility from the United States. The top U.S. envoy for North Korea, Glyn Davies, had warned Pyongyang on Monday that a possible third nuclear test would be "a serious miscalculation and mistake." An unidentified North Korean Foreign Ministry official suggested that a nuclear test had not originally been part of the regime's plans. "We did not envisage such a military measure as a nuclear test as we planned to launch a scientific and technical satellite for peaceful purposes," he said in a report Tuesday by the state-run Korean Central News Agency. The rocket launch in April was timed to coincide with the 100th anniversary of the birth of Kim Il Sung, the founder and longtime leader of North Korea. It also came a few months after the death in December of Kim Jong Il, who had headed the secretive regime since 1994. His son and chosen successor, Kim Jong Un, has taken over as "supreme leader" of the nation, but the level of his influence on policy decisions remains unclear. CNN's Paula Hancocks and Jethro Mullen contributed to this report.