The US Treasury Department announced new sanctions Thursday targeting 13 companies it said were part of a network supporting the sale of Iranian oil and petrochemicals.

Based in China, Hong Kong and the United Arab Emirates, the designated companies are accused of facilitating the sale of hundreds of millions of dollars’ worth of Iranian petrochemicals and petroleum products to buyers in East Asia on behalf of previously sanctioned Iranian companies, including the National Iranian Oil Company.

“Today’s action further demonstrates the complex sanctions evasion methods Iran employs to illicitly sell petroleum and petrochemical products,” Brian Nelson, the Treasury's under secretary for terrorism and financial intelligence, said in a statement. “The United States will continue to implement sanctions against those actors facilitating these sales."

The Biden administration has stepped up its targeting of Iran’s sanctions evasion efforts as prospects for resurrecting the landmark nuclear deal have dimmed and anti-government protests continue across Iran. Since June, the administration has imposed five rounds of sanctions on Iran’s illegal petroleum and petrochemical trade.