A senior European official directly involved in the diplomacy also welcomed the decision to send Mr. Burns, the State Department’s third-ranking official, calling it a “major change” in American policy.

Dana M. Perino, the White House press secretary, said it was Secretary of State Condoleezza Rice who had approached the president about sending Mr. Burns. One official, speaking on condition of anonymity, said that Ms. Rice had decided to test Iran’s willingness to consider an international package of incentives meant to coax Iran into making concessions on its nuclear program.

The combination of diplomacy and increasing sanctions, including those by the European Union against Iran’s largest bank last month, had produced some signals within Iran that it might being softening its stance, and Ms. Rice “decided it was a chance to press the advantage,” the official said. Vice President Dick Cheney and other senior aides discussed the idea as well, said the official, who was not identified because he was speaking about internal discussions.

The extent of American involvement remains unclear. Mr. McCormack described Mr. Burns’s participation in the talks as “a one-time-only deal.” Ms. Perino would not rule out additional meetings with the Iranians, saying it depended on the outcome of the meeting.

Some administration officials have even discussed whether to post American officials in Iran without established diplomatic relations, as in Cuba, but have said a decision has not yet been made.

The presence of an American at the talks this weekend may help quiet the mounting calls in both the United States and Israel for military strikes against Iran because of its recent expansion of its uranium enrichment program and its unwillingness to fully explain its suspicious past nuclear activities.

In Tehran, Ayatollah Ali Khamenei, Iran’s supreme leader, said Wednesday that his country had “clearly defined red lines” that had to be respected in negotiations, a reference to Iran’s insistence that it has the right to peaceful nuclear energy.