WASHINGTON - In a break with past Bush administration policy, a top U.S. diplomat will for the first time join colleagues from other world powers at a meeting with Iran's chief nuclear negotiator, The Associated Press has learned.

William Burns, America's third highest-ranking diplomat, will attend talks with the Iranian envoy, Saeed Jalili, in Switzerland on Saturday aimed at persuading Iran to halt activities that could lead to the development of atomic weapons, a senior U.S. official told the AP on Tuesday.

Official contacts between Iran and the United States are extremely rare and although Washington is part of a six-nation effort to get Iran to stop enriching and reprocessing uranium, the administration has shunned contacts with Tehran on the matter.

The senior U.S. official, who spoke on condition of anonymity ahead of a formal announcement of Burns' plans expected on Wednesday, acknowledged a shift in the administration's approach but stressed that Burns would not meet Jalili separately and would not negotiate with him.

"This is a one-time event and he will be there to listen, not negotiate," the official said.