The chief U.S. nuclear negotiator says it'll be difficult to clear suspicions on whether North Korea pursued a uranium program.

Speaking at the American Chamber of Commerce in Seoul, Christopher Hill called on Pyongyang to explain the allegations. He said that if there is related equipment, the North should say how it will be disposed of.

Hill said that talks with North Korea on the matter have not yielded a solution, but he hoped to resolve it within the year.