North Korea threatened Saturday to withdraw from peace talks for the Korean Peninsula because the United States refused to consider discussion of an American troop withdrawal from South Korea.

The United States blamed North Korea for the failure of the four-country talks to finish laying the groundwork for full negotiations on a permanent peace treaty. A treaty would replace the armistice that ended fighting in the 1950-1953 Korean War.China, the United States and North and South Korea took part in the talks that began Monday.

South Korea insisted the negotiations had made "modest progress."

"These talks are better than no talks," said South Korean delegation chief Song Young-shik.

The peace talks are running behind the schedule the four countries set for themselves in December. The plan was to organize the talks last month in Beijing, but the North Koreans canceled that meeting.

China's mediator said both the United States and North Korea were responsible for the deadlock, but that he expected a new round of talks would be held this summer. The negotiations are widely expected to drag on for years.

The United States maintains 37,000 U.S. troops in South Korea, across the border from North Korea and its 1.2 million-strong military. The United States says the very presence of the troops helps assure peace and stability.

The United States maintains that U.S. troop deployments should have nothing to do with the talks, but North Korea's deputy foreign minister, Kim Gye Gwan, said they are the only foreign troops on the peninsula and are a fundamental issue.

"We have no more interest if the four-party talks are not going to address the fundamental issues," Kim said.

Assistant Secretary of State Stanley Roth, the head of the U.S. delegation, said North Korea appeared unwilling to make "concrete progress towards peace,"

"We proposed to negotiate steps to reduce tensions and build confidence on the Korean peninsula," Roth said. "Unfortunately, the North Korean delegation was not prepared to do so."