The United States has sanctioned China’s state-run oil company and two other foreign energy firms for trading with Iran.

The companies subject to U.S. sanctions are Zhuhai Zhenrong of China, which is Iran’s largest refined petroleum supplier, Kuo Oil of Singapore and FAL Oil of the United Arab Emirates (UAE).

The U.S. State Department says the three firms will not be eligible for export approval by the U.S., financing by the U.S. Export-Import Bank and loans worth ten-million dollars or more by a U.S. financial institution.

The recent U.S. sanctions are to pressure Iran, which is suspected to be secretly developing nuclear weapons.