North Korea has conducted three tests of nuclear devices underground since 2009. The most recent test was in February. Nuclear tests have coincided with long-range rocket deployments, though North Korea is not suspected of having the ability to equip a missile with a nuclear warhead.
A report submitted by the South Korean Defense Ministry to a parliamentary committee Tuesday said Pyongyang has made substantial progress on its nuclear program.
"[North Korea's nuclear program] was at a developmental and experimental stage till 2010, but it has developed into a real threat in 2013 that can actually be weaponized and used at any time," China's official Xinhua News Agency quoted the report as saying.
Chinese President Xi Jinping said he was eager to bring North Korea back to multilateral nuclear negotiations with South Korea, China, Japan, Russia and the United States. Nuclear negotiations started in 2003 but have been suspended since 2008.
South Korean President Park Geun-hye said last week she was frustrated with Pyongyang's decision to pursue nuclear weapons despite widespread international condemnation. She called on Pyongyang to disarm.