(CNN) -- North Korea has more uranium enrichment facilities than it has admitted to previously, a U.S. scientist charged Thursday.
Prof. Siegfried Hecker of Stanford University told CNN his conclusion is based on his study of recent satellite images and other research, and what he saw when he was invited by North Korea to visit its Yongbyon nuclear power plant in 2010 to see its secret uranium enrichment program.
"When I saw the sophistication and scale of that uranium enrichment facility at Yongbyon in a building that I had been in before that housed something totally different, it was clear that they started the program long before the time that they had said, which was April 2009. So my conclusion was, they had to have another site someplace else," said Hecker.
The Stanford University scientist visited Yongbyon in November, 2010, and reported that he saw a facility that housed 2,000 centrifuges and was producing low-enriched uranium. It could, however, he wrote then, "be readily converted to produce highly-enriched uranium (HEU) bomb fuel."
Hecker, an emeritus director of Los Alamos National Laboratory, also estimated that North Korea has between four and eight plutonium bombs, each of which could do damage similiar to that of the atomic bomb dropped on Nagasaki in Japan at the end of World War II.
He is in South Korea attending international nuclear conferences ahead of the Seoul Nuclear Security Summit next week.
North Korea, although not officially on the agenda, will be discussed in bilateral meetings at the summit, which will be attended by more than 50 heads of state, including U.S. President Barack Obama.
Hecker's assessment comes just weeks after the United States agreed to a deal with North Korea in which Pyongyang would allow United Nations weapons inspectors back into the country in exchange for food aid. North Korea ejected inspectors from the U.N.'s International Atomic Energy Agency from its Yongbyon nuclear complex in April 2009, about a month before it conducted its second nuclear weapons test.
However, per the new agreement, the inspectors will only have access to the sites to which North Korea has admitted having, not the ones Hecker suspects are still undisclosed.
As part of the deal, Pyongyang also agreed to refrain from nuclear tests and long-range missile tests, but just last week announced it would launch a rocket with a satellite attached in mid-April.
"I wasn't so surprised that they would do a space launch," said Hecker, who is also the co-director of the Center for International Security and Cooperation. "What I was surprised at is, first of all they even agreed to a missile moratorium but then immediately turned around to say, 'We're doing this space launch' and quite frankly, that makes a mockery of the agreement."
North Korea says it has a right to a peaceful space program and invited international space experts and journalists to witness the launch, the state-run Korean Central News Agency said last week. The satellite would be sent into orbit in mid-April to mark the 100th anniversary of the birth of Kim Il Sung, North Korea's founder and current leader Kim Jong Un's grandfather.