Mr Hwang defected in 1997

Mr Hwang, a former secretary of North Korea's ruling Workers Party, was the chairman of a research institute run by South Korea's National Intelligence Service (NIS).

He had been living with fellow defector, Kim Duck-hong, in a safe house under the protection of the NIS which says they are at risk of assassination.

"We have told Hwang about his dismissal. He and his deputy, Kim Duck-hong, were also asked to move out of a safe house," a spokesman said.

He added that Mr Hwang - the most senior North Korean official to defect - would continue to receive state subsidies and protection.

Critic

Mr Hwang, 77, once a close confidant of North Korean leader Kim Jong-il, has been a fierce critic of his regime since defecting in 1997.

We do not like the fact the NIS is evicting us from the safe house, but have no alternative

Kim Duck-hong

He has also denounced South Korean President Kim Dae-jung for his policy of engagement with the North.

But correspondents say that with the recent thawing of relations between the two Koreas, his views have become an embarrassment.

Assassination fears

A senior NIS official said Mr Hwang's activities had been restricted due to safety concerns.

Kim Jong-il needs aid from the South

"We have secured intelligence information through unofficial channels that North Korea may use ethnic Koreans in China to eliminate Hwang," he added.

But in a newspaper interview on Thursday, Kim Duck-hong accused the government of breaking a pledge to guarantee their security and freedom.

"North Korea cannot easily commit terrorist acts against us because they are trying to take off their terrorist cap... to secure economic recovery," he added.

Relations on the Korean peninsular, which split in two in 1945, have improved markedly since a historic summit in June.

But Mr Kim warned North Korea's military was still powerful enough to invade and destroy South Korea "two to three times over".

"It's absolutely wrong to think that North Korea is weak. It could very well invade South Korea if American troops withdraw from the South," he added.