US President George W. Bush has written to North Korea's leader Kim Jong-il to say Pyongyang must fulfil its promise to reveal all details of its nuclear programmes, the White House said yesterday.

The unusual direct communication between Mr Bush and the communist leader he has professed to loathe was made amid uncertainty over when and how Pyongyang will meet nuclear disarmament steps agreed with Washington.

"The President reiterated our commitment to the six-party talks and stressed the need for North Korea to come forward with a full and complete declaration of their nuclear programs, as called for in the September 2005 six-party agreement," White House spokesman Gordon Johndroe said.

North Korea shuttered its main reactor in July under a February deal. In exchange for disabling its plutonium production facilities, the impoverished country will receive one million tons of heavy fuel oil or equivalent aid.

As part of the accord reached earlier this year, Pyongyang, which tested a nuclear device last year in defiance of international warnings, must also provide a complete accounting of its nuclear programmes by the end of the year.

Both the United States and North Korea made clear on yesterday they could accept some slippage in that deadline.