US Secretary of State Condoleezza Rice says differences remain among the UN Security Council's five permanent member countries, plus Germany, on the subject of implementing further sanctions against Iran.
Diplomats from the Council's veto-wielding members and Germany did not resolve their disagreements during a conference call on Thursday.
But Rice says they are united on a two-track strategy that calls for sanctions if Iran continues to enrich uranium and benefits if it stops the process.
US Under Secretary of State for Political Affair Nicholas Burns took part in the conference call with his counterparts from Britain, France, Russia, China and Germany (the so-called P-Five-Plus-One).
The diplomats are attempting to finalize terms of a third UN sanctions resolution against Tehran. But US officials have said there is no chance of submitting a new resolution before January.
In other news, Russia this week began delivering fuel to Iran's Bushehr nuclear power plant on the Persian Gulf, under terms that mandate the return of all spent fuel to Russia.
A spokeswoman (Irina Yesipova) for the Russian state contractor (Atomstroiexport) building the plant tells VOA fuel will be delivered from Russia under strict supervision by the International Atomic Energy Agency.
The head of Atomstroiexport, Sergei Shmatko, said Thursday construction of the plant will be completed, but it will not be "launched" before late next year.
The United States and its Western allies have accused Iran of attempting to develop nuclear weapons.
Tehran argues that its nuclear work is strictly for civilian purposes.