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

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

Official contacts between Iran and America 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 American official, who spoke on condition of anonymity ahead of a formal announcement of Mr. Burns’s plans expected tomorrow, acknowledged a shift in the administration’s approach but stressed that Mr. Burns would not meet Mr. 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.

The meeting in Geneva is being led by the European Union foreign policy chief, Javier Solana, who is seeking a definitive answer from the Iranians to an offer of incentives that the five permanent members of the U.N. Security Council and Germany presented last month.

The package of incentives was accompanied by a letter from Secretary of State Rice and the foreign ministers of the other five countries and sets out a scenario in which Iran would get a temporary reprieve from crippling economic and financial sanctions in exchange for freezing its enrichment activities.

Preliminary negotiations over a permanent halt could then begin, although America would not join them until after Iran agrees to fully suspend uranium enrichment, which can produce the fuel needed to make nuclear bombs.

The senior American official said Mr. Burns would be at the meeting with Mr. Jalili to demonstrate the unity of the countries making the offer of incentives but also to reinforce Ms. Rice’s signature on the letter from the foreign ministers.

But Mr. Burns will also “reiterate that our terms for negotiations remain the same: Iran must suspend its enrichment and reprocessing activities,” the official said.

Iran last week responded to the offer through the European Union but indicated that it has no plans to stop enriching uranium — a key demand. The process can be used to generate electricity or build nuclear weapons.

Iran says its nuclear program is purely energy-related, but America accuses it of trying to develop atomic weapons.