The Trump administration has said it had imposed sanctions on an independent “teapot” refinery in China for buying billions of dollars’ worth of Iranian oil, as Washington and Tehran head into another round of peace talks over the weekend, Reuters reports.

The Treasury Department targeted Hengli Petrochemical (Dalian) Refinery, which it said is one of Iran’s largest customers of crude oil and petroleum products.

The department’s Office of Foreign Assets Control said it also imposed sanctions on about 40 shipping companies and vessels that operate as part of Iran’s shadow fleet.

China has said it opposes “illegal” unilateral sanctions.