The Treasury Department said it was targeting Hebei Xinhai Chemical Group and three port terminal operators in Shandong province which it said had been involved in handling hundreds of millions of dollars worth of Iranian oil.

"The United States remains resolved to intensify pressure on all elements of Iran's oil supply chain to prevent the regime from generating revenue to further its destabilizing agenda," Treasury Secretary Scott Bessent said in a statement.

The State Department said it was the third US action against a so-called teapot, or small-scale, refinery in China since early February when President Donald Trump issued an order to increase pressure on Iran.

Trump, however, has also said that he is acting reluctantly and that he is hopeful for a deal on Iran's nuclear program that averts a threatened Israeli military strike.

Both Iran and Trump's envoy, Steve Witkoff, have described initial talks as positive.

Tehran, which wants sanctions relief, has lashed out at the renewed US pressure, saying it went against efforts to reach a diplomatic solution.

Trump in his first term withdrew from a previous nuclear deal with Iran and imposed sweeping sanctions, including pressing all other countries -- of which China is the most significant -- to stop buying Iranian oil.