WASHINGTON - President Bush has authorized the most significant U.S. diplomatic contact with Iran since the Islamic Revolution in 1979, sending the State Department's third-ranking official to Geneva for a meeting this weekend on Iran's nuclear program, administration officials said Tuesday.

The decision appeared to bend, if not exactly break, the administration's insistence that it would not negotiate with Iran over its nuclear programs unless it first suspended uranium enrichment, as demanded by three resolutions of the U.N. Security Council.

Still, after months of accusations and counteraccusations from the United States and Iran, the meeting raised the prospect of an intensified diplomatic push to resolve concerns over Iranian nuclear activity, not unlike the lengthy and painstaking talks that resulted in a deal last month with North Korea.

William J. Burns, the undersecretary of state for political affairs, will attend a meeting Saturday with the European Union's foreign policy chief, Javier Solana, and Iran's nuclear negotiator, Saeed Jalili, a senior administration official said, speaking on condition of anonymity ahead of an official announcement today. The Associated Press also reported that Burns will go to Geneva.

Jalili is expected to present Iran's formal response to economic and diplomatic incentives that Germany and the Security Council's five permanent members, Russia, China, France, Britain and the United States, presented in June. Representatives from those countries will also attend the meeting.