Washington issued the warning during a meeting with South Korea and Japan in Seoul last week, the Asahi Shimbun newspaper said, quoting Japanese government sources.

"If there is no progress at this time, (the United States) will not allow talks to continue," a Japanese official was quoted as saying in the daily.

President George W. Bush's administration was likely to adopt a stronger stance against North Korea if the United States withdraws from the six-party talks due to start in the final week of July, the daily said.

"The Bush administration has no plans to stay in a round of marathon talks that produce no achievement," the official was quoted as saying.

"If no end result is seen by the end of the year, the United States will likely ask concerned parties to join in taking tougher measures" against North Korea.

After more than a year of stalemate, North Korea agreed earlier this month to return to the six-way talks, which also involve South Korea, Japan, Russia and China, Pyongyang's closest ally.

The first round of talks was held in Beijing in August 2003, nearly a year after North Korea allegedly told US officials in October 2002 it was running a uranium enrichment program.

Meanwhile, South Korean cable news network YTN said Monday China hopes to begin hosting the upcoming six-party talks on July 26 but the report gave no date for when the meeting would end.

Ahead of the July six-way talks, US Secretary of State Condoleezza Rice visited China, Japan and South Korea and urged North Korea to renounce its nuclear weapons for good.

But the North wants a step-by-step approach to weaning itself off its nuclear program, fearing it could come under attack by the United States. The CIA believes Pyongyang has at least one or two crude nuclear bombs.