U.S. UN Ambassador Susan Rice said Thursday that "pressure is mounting," but "Iran still has a choice" on the nuclear issue.

Rice made the statement to reporters that the United States is working "constructively" with other countries in the six- nation mechanism, which groups the five permanent members of the UN Security Council -- Britain, China, France, Russia and the United States -- plus Germany.

The United States is leading a push to press for new sanctions against Iran on the ground the Middle East country is developing clandestine nuclear program, but Tehran insists that its nuclear program is for peaceful purpose.