North Korea's surprise disclosure of an ultra-modern uranium enrichment facility is raising fresh questions about the ability of US intelligence to penetrate one of the world's most reclusive states. Are there other uranium enrichment facilities in North Korea that Washington may not know about? How extensive is the proliferation of sensitive nuclear technology? What are the motives for the North's nuclear “show-and-tell”? US officials have played down the significance of the revelations, saying intelligence agencies have known for years about North Korea's uranium enrichment efforts. Still, Defense Secretary Robert Gates, a former CIA director, has acknowledged he did not know about the enrichment facility that North Korea unveiled to a visiting US nuclear scientist this month. Other officials have declined comment, citing sensitive intelligence. Instead of being buried deep in a mountain, the facility was located at the Yongbyon nuclear complex – a well-known site under close scrutiny by US spy satellites monitoring developments in the North's nuclear drive. “From an intelligence perspective, it's sort of your worst nightmare,” said Victor Cha, director of Asian Affairs at the White House National Security Council during the Bush administration. “Their capabilities are beyond what (US intelligence) thought they would be and it was a facility that was basically sitting right under our noses, but we weren't able to see it.” US officials have cautioned for years about the difficulties of spying on North Korea and its secretive leader Kim Jong-il. Passing US satellites can be evaded, the mountainous landscape helps conceal troops and citizens are encouraged to report on each other. “On top of that, a nuclear weapons program is clearly the holy of holies, and the most secret of secrets,” said Bruce Klingner, former head of the CIA's Korea branch. That raises questions about the North's motives for unveiling the facility. Admiral Mike Mullen, the top US military officer, linked it to Kim Jong-il's attempts to boost his son's military credentials before an eventual succession. Analysts have questioned whether the impoverished state might be using uranium enrichment as a bargaining chip in stalled aid-for-disarmament talks. But the top US State Department official for the region warned Congress in September that Washington lacked intelligence on North Korea, describing it as a “black box.” The United States has some glimpses into North Korea “but the truth is often times in retrospect, some of that intelligence has proven to be wrong,” said Kurt Campbell, assistant secretary of state for East Asia and Pacific Affairs. “It's a very, very hard target, probably the hardest we face in the global arena,” he said.