The United States is calling on Iran to negotiate over its nuclear program, after diplomats agreed to resume talks in two weeks on suspending uranium enrichment.

Saturday's talks were the first to include the United States.

Just before the talks began, Iran ruled out freezing its enrichment program. In Washington, State Department spokesman Sean McCormack said Iran has two weeks to "give a clear answer."

Washington's participation had raised expectations of possible compromise on a formula under which Iran would agree to stop expanding its enrichment activities. In exchange, six powers _ the five permanent U.N. Security Council members and Germany _ would hold off on passing new U.N. sanctions.