"The United States is today sanctioning an international network facilitating the shipment of millions of barrels of Iranian crude oil worth billions of dollars to China," US State Department spokeswoman Tammy Bruce said in a statement.

The sales, she added, were being conducted "on behalf of Iran's Armed Forces General Staff (AFGS) and its front company, Sepehr Energy."

The sanctions follow similar designations in recent weeks, at the same time as Washington and Tehran have stepped up nuclear talks.

The two sides held their fourth round of indirect talks over the weekend, which mark their highest-level contact since the United States in 2018 pulled out of the 2015 nuclear deal.

Following the talks, Iran said it is open to accepting temporary limits on its uranium enrichment.

Iran currently enriches uranium to 60 percent purity -- far above the 3.67 percent limit set in the 2015 deal but below the 90 percent needed for weapons-grade material.