Hwang is the most prominent North Korean defector
A high-ranking North Korean official who defected to the South six years ago has told US congressmen that Pyongyang cannot be trusted to stick to a new nuclear weapons deal.
Hwang Jang-yop, who was secretary of the North Korean Workers' Party, was speaking to the House of Representatives' Committee on Homeland Security during his first visit to Washington.
Mr Hwang's comments came as Chinese and North Korean media reported that Pyongyang had agreed "in principle" to further talks on its controversial nuclear programme.
He said no significance should be attached to any promise by the North Korean leader, Kim Jong-il.
The committee chairman, Chris Cox, said Mr Hwang's testimony had helped them understand the horrible mistreatment of the North Korean people.
"He has seen Kim Jong-il grow up. He is intimately familiar with
the workings of the North Korean state and the North Korean
society," said Mr Cox, a Republican.
During his visit to the US, Mr Hwang is also meeting State Department officials to brief them on North Korea.
Mr Hwang, a former tutor to the North Korean leader, has been kept under heavy guard by the Seoul government since he defected during a visit to China in 1997.
Earlier this year, he said he was told in 1996 that North Korea had already developed nuclear weapons.