A US monitor confirmed on Tuesday that North Korea is fulfilling its pledge to dismantle its nuclear test site as shown by satellite images. In a move welcomed by Washington and Seoul, North Korea said at the weekend it will "completely" destroy the Punggye-ri test site, in a ceremony scheduled between May 23-25 in front of invited foreign media. But no observers from international atomic monitoring agencies have been invited, raising concerns over the openness of the process. Punggye-ri, in the northeast of the country, has been the site of all six of the Norths nuclear tests, the latest and by far the most powerful in September last year, which Pyongyang said was an H-bomb. The respected 38 North website said Tuesday that satellite images dated May 7 showed "the first definitive evidence that dismantlement of the test site was already well under way". Several key operational buildings as well as smaller sheds had been razed and rails connecting the tunnels to their waste piles were removed, the monitoring group said. Excavation of a new tunnel has also been halted since late March, it added. Images showed preparatory work for the destruction ceremony had also begun, including a newly positioned foundation among the waste piles believed to have been built for the invited journalists. "It is conceivably for a future camera position to record the closure of the West Portal," the group said. However no tunnel entrances appear to have been permanently closed and some main buildings are still intact, it added, saying the destruction of those facilities was likely to be carried out in front of the foreign media. Roh Jae-cheon, spokesman for South Koreas Joint Chiefs of Staff, said the Souths military has seen signs that the North is conducting "preparatory work" so that it can carry out its plans to shut down the testing ground. He did not specify what the military has seen. The dismantling of the site will precede a historic summit between North Korean leader Kim Jong Un and US President Donald Trump in Singapore on June 12. Pyongyang pledged to close the testing ground after Kim last month declared the countrys nuclear force complete and said it had no further need for the complex. Kims latest diplomatic overture has seen him hold a summit with the Souths President Moon Jae-in and travel twice in less than two months to meet with Chinese leader Xi Jinping. The two Koreas are due to meet for a high-level meeting on Wednesday to discuss follow-up measures from their summit last month, Seouls unification ministry said. Not everyone shared the optimism over Pyongyang’s nuclear pledge. A top defector said that North Korea will never completely give up its nuclear weapons. The current whirlwind of diplomacy and negotiations will not end with "a sincere and complete disarmament," but with "a reduced North Korean nuclear threat," said Thae Yong-ho, who fled his post as the Norths deputy ambassador to Britain in August 2016. "In the end, North Korea will remain a nuclear power packaged as a non-nuclear state," Thae told the Souths Newsis news agency. Pyongyangs sudden change in attitude was probably driven by the mounting international sanctions, which have included measures hitting sectors including coal, fish, textiles and overseas workers, he said. But it had a long history of making overtures that ultimately came to nothing, he warned. "North Koreas diplomacy has always been a repeat of hardline and appeasement," Thae added. "It is North Koreas diplomatic tactic to push the situation to extreme confrontation and suddenly send peace gestures."