U.S. Secretary of State Condoleezza Rice says a senior American envoy is joining Saturday's nuclear talks with Iran to show Washington's firm support for a diplomatic resolution of the dispute with Tehran.

Rice says the United States hopes its position is clear to Iran during the talks in Geneva.

Speaking to reporters in Washington Thursday, Rice said Undersecretary of State William Burns is at the meeting to show that the U.S. and other world powers are united in their efforts to end the standoff with Iran.

In announcing on Wednesday that Burns will be at the meeting between Iran's top nuclear negotiator Saeed Jalili and the European Union's foreign-policy chief Javier Solana broke with long-standing U.S. policy.

Iran's foreign minister, Manouchehr Mottaki, visiting Syria Thursday, says U.S. participation will add a positive aspect to the talks.

French Foreign Minister Bernard Kouchner also welcomed the U.S. presence, although he says he does not expect the meeting to produce significant results.

White House spokeswoman Dana Perino says Burns' trip will be a "one-time" occurrence to demonstrate Washington's commitment to resolving the nuclear dispute diplomatically. She stressed Wednesday that Burns will be at the meeting to "listen," not to negotiate with Iran.

The U.S. has previously refused to attend any talks with Tehran about Iran's nuclear program until it stops sensitive nuclear activities.

The five permanent members of the United Nations Security Council plus Germany have called on Iran to halt its uranium enrichment procedures, which could produce weapons-grade nuclear material. Iran says its nuclear program is peaceful.

Some information for this report was provided by AFP, AP and Reuters.