Rep. Park Sun-young of the Liberty Forward Party said the underground facility on North Korea's west coast has been operating since it was built between 2001 and 2006, Yonhap News Agency said.
To support her claim, Park cited a North Korean military official she said headed security at the construction site.
"North Korea has already been developing nuclear weapons using enriched uranium since 2007, switching from its plutonium production program," Park said during a parliamentary session. "But the [South Korean] government has not been aware of this fact."
When highly enriched, uranium can become weapons grade, which would give the North Korean regime another way of building a nuclear device besides using plutonium, the South Korean news agency said.
Park said it was only after the new plant went into production that North Korea invited New Mexico Gov. Bill Richardson to Pyongyang last year and expressed a willingness to put the Yongbyon complex under international monitoring. North Korea expelled International Atomic Energy Agency monitors in 2009 after the U.N. Security Council condemned a rocket launch that was considered a long-range missile test.
"The Yongbyon facility has been almost emptied in preparation for international monitoring," Park said. "South Korea and the U.S. have continuously been fooled by North Korea."