US President George W Bush has sent a personal letter to the North Korean leader Kim Jong-il, the White House has confirmed.
North Korea is in the process of disabling its Yongbyon reactor
The letter urged Pyongyang to honour its pledge to reveal all the details of its nuclear programme.
The letter was delivered by US envoy Christopher Hill.
Mr Hill has just visited North Korea to assess progress on the disabling of its main nuclear reactor at Yongbyon as part of an aid-for-disarmament deal.
US National Security Council spokesman Gordon Johndroe said that Mr Bush had written to all leaders involved in the six-party talks about North Korea's nuclear programme.
"In these letters, 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 programmes, as called for in the September 2005 six-party agreement," he said.
The letter was handed over during Mr Hill's meeting on Tuesday with North Korea's foreign minister Pak Ui-chun.
Under an international deal, signed in February, Pyongyang pledged to declare all its nuclear programmes and disable Yongbyon by the end of the year.
Deal 'at a crossroads'
News of the letter comes a day after Mr Hill and South Korean ministers expressed concern that North Korea's declaration of its nuclear programme might not be completed on time.
Mr Hill said this declaration had to include details of the country's uranium enrichment programme.
"We want to make sure that when we do transfer, even the first draft, it is a credible effort," he said in Beijing.
"We need them to step up and show some trust in us and trust in the process," he said.
South Korean Foreign Minister Song Min-soon said on Thursday that the deal was "at a crossroads where it may proceed towards a stable phase or to a rough road".
The US has said it would consider removing North Korea from a list of countries it says sponsors terrorism if Pyongyang fully complies with the deal.
Meanwhile negotiations are continuing over a date for the next round of six-party talks, between the Koreas, Russia, Japan, the US and China.
Talks were due to restart this week but were called off because of to "scheduling problems", according to a US State Department spokesman.
Mr Bush once branded North Korea part of the "axis of evil", along with Iran and pre-war Iraq, and said the country was "an oppressive regime [whose] people live in fear and starvation".
North Korea's nuclear ambitions first attracted international attention in 2002 and in October 2006 Pyongyang shocked the world by testing a nuclear missile.