The U.S. on Tuesday imposed sanctions on a shipping network it says has sent millions of barrels of Iranian oil to China, the State Department said, days after Iran and U.S. negotiators concluded a fourth round of nuclear talks in Oman.

The United States on Monday sanctioned three Iranians and a related technology firm on accusations of advancing Tehran's nuclear weapons research.

Nadergholi Ebrahimi has asked Supreme Leader Khamenei to rethink Iran's nuclear policy if negotiations with the U.S. collapse.