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.

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

Iran condemned the United States for imposing new sanctions on its nationals and a company, calling the move unjust and contradictory as both sides pursue indirect negotiations over Tehran’s nuclear program and the anti-Iran sanctions.