Pyongyang also insisted at the start of peace talks in Geneva on Thursday that any peace pact should be signed only between the United States and North Korea, leaving rival South Korea out, Yonhap News Agency said.

The latest reiteration of the decades-old demand, made in a keynote address by the North's chief delegate to the four nation peace talks, appeared to scuttle any hope of progress.

``The (United States) will have to make a courageous decision on US troops withdrawal and the signing of a North Korea-US peace treaty,'' Vice Foreign Minister Kim Gye-Gwan told delegates, according to Yonhap.

Kim went on to blame the US for the lack of progress in five earlier rounds of the talks, saying the North had made efforts to produce results. ``Patience has its own limit and we doubt whether to continue the Four-party peacetalks,'' he added.

The two-year-old talks have consistently failed to produce results following the outright rejection by the United States and its ally South Korea to discuss demands for a troop withdrawal and bilateral peace pact.

A US official said on Thursday it would not discuss the issue with Pyongyang. ``We will not discuss troop withdrawal with anyone other than our allies,'' he said, requesting anonymity.

The United States has a 37,000-strong force in South Korea to ward off any possible attack from North Korea which invaded the South in June 1950, sparking the Korean War.

The sixth round of four-party talks -- which include both Koreas and their respective Cold War allies the United States and China -- opened in Geneva on Thursday.

The start of the round was overshadowed by fears that the unpredictable North plans to launch a new long-range missile which could hit parts of the United States and shatter regional stability.

The four-party talks, launched in December 1997, aim to reducetensions and signal formal peace treaty to replace the current armistice which has held the peace since 1953.