Kim Seung-Gyu, head of the National Intelligence Service, also reportedly told parliament that the North might carry out further nuclear tests following Monday's one.

"North Korea is believed to have stored some 30 to 40 kilograms of plutonium," he was quoted as telling parliament's intelligence committee.

Chung Hyung-Keun, an opposition Grand National Party lawmaker who serves on the committee, quoted the intelligence chief.

The 40 kilograms includes 10-12 kilograms that it had secured before it opened its nuclear facilities in Yongbyon, 90 kilometers (56 miles) north of Pyongyang, to International Atomic Energy Agency inspectors, Kim said.

"As one bomb needs five to six kilograms of plutonium, North Korea would be able to make up to seven atomic bombs," Chung told journalists.

"We cannot rule out the possibility of the North carrying out further nuclear tests as Pakistan, for example, carried out six nuclear tests," he said.

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