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

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 announced new sanctions Monday against Iran over its nuclear program, despite ongoing negotiations between the two countries over the sensitive issue.