The U.S. Treasury Department on Tuesday imposed sanctions on more than 20 companies in a network that it said has long sent Iranian oil to China, days after negotiators from Iran and the United States concluded a fourth round of nuclear talks.

WASHINGTON (Reuters) - 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.