WASHINGTON (CNN) -- The United States on Friday sought clarification from Iran in response to its counter-offer involving shipping low-enriched uranium for refining abroad, senior administration officials said.

The apparent Iranian refusal of a nuclear proposal centers on Iran not wanting to send its uranium to Russia, as stated in a document agreed on earlier this month at the headquarters of the International Atomic Energy Agency (IAEA) in Vienna, Austria.

Tehran is proposing instead that its uranium be enriched on Iranian soil by a third country, under IAEA supervision, Obama administ