North Korea says it will 'expand' nuclear program in face of U.S. 'hostility' May 23, 2012 -- Updated 0512 GMT (1312 HKT) A North Korean soldier stands guard in front of an Unha-3 rocket at Tangachai -ri space center on April 8, 2012. An analysis of satellite images shows increased activity at North Korea's nuclear site Pyongyang says it will "expand and bolster" its nuclear program It says it is responding in self-defense to "the hostile policy of the U.S." G8 leaders have warned North Korea it faces further isolation over its nuclear program Seoul, South Korea (CNN) -- North Korea has said it will press on with its nuclear program as a response to what it described as hostility from the United States after an analysis of satellite images indicated increased activity at its nuclear test site. "We had access to nuclear deterrence for self-defense because of the hostile policy of the U.S. to stifle the DPRK by force and we will expand and bolster it nonstop as long as this hostile policy goes on," an unidentified spokesman for the North Korean Foreign Ministry said in a report Tuesday by the state-run Korean Central News Agency. DPRK is short for Democratic People's Republic of Korea, North Korea's official name. The top U.S. envoy for North Korea, Glyn Davies, warned Pyongyang on Monday that a possible third nuclear test would be "a serious miscalculation and mistake." The North Korean official suggested Tuesday 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. But the statement concluded with a vague threat: "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 counter-measures for self-defense." North Korea launched a rocket on April 13, which failed less than two minutes into the flight. It said the launch was to put a satellite into orbit, but much of the international community saw it as a cover up for testing its ballistic missile technology. The move torpedoed a deal reached in February under which Pyongyang agreed to suspend its nuclear activities in exchange for food aid shipments from the United States. Many analysts assume an atomic test by North Korea is just a matter of time following the failure of the rocket launch last month. Two previous rocket launches in 2006 and 2009 were followed weeks or months later by nuclear tests. And activity is being ramped up at North Korea's nuclear test site, a sign that the country is preparing for a test, according to an analysis of new satellite images by the defense publication IHS Janes. Mining carts and excavation equipment at Punggye-ri's tunneling area can be seen in satellite images taken by Digital Globe and GeoEye in the past month. Earth and debris are being removed from the tunnel in the largest quantities seen so far, according to the Janes assessment. An image from mid-April shows a full mining train, including an engine and several carts, outside of the tunnel. And a more recent shot on May 9 reveals new road networks at the site along with carts and a vehicle at the facility. In the statement Tuesday, North Korea denounced as "absolutely intolerable" a declaration from the Group of 8 leaders over the weekend, which stated that Pyongyang faces more isolation if it continues its pursuit of a nuclear program. CNN's Jennifer Rizzo contributed to this report