U.S. Secretary of State Condoleezza Rice says the decision to send a top envoy to nuclear talks with Iran is a "strong signal" the United States is serious about diplomacy with Tehran. 

However, Rice says the move to send Under Secretary of State William Burns to the talks Saturday in Geneva does not soften the key U.S. demand that Iran stop enriching uranium. 

She says she hopes the U.S. gesture will encourage Iran to accept an international incentives package and halt its controversial nuclear work. The attendance by Burns will mark a shift in U.S. policy. 

The Bush administration has long insisted on shunning nuclear talks with Iran until it stops enriching uranium. Iran's Foreign Minister Manouchehr Mottaki says U.S. participation at the talks is a positive development, and that he expects progress.