The U.S. State Department has imposed sanctions on three energy trading companies for selling refined oil and gasoline to Iran, including against a Chinese company it says is Iran's largest supplier of refined petroleum.

Officials identified the three companies sanctioned Thursday as China's state-run Zhuhai Zhenrong Company, Singapore's Kuo Oil, and the United Arab Emirates' FAL Oil.

The announcement comes just days after U.S. Treasury Secretary Timothy Geithner went to China for talks aimed at persuading Chinese officials to support U.S. sanctions against Iran's oil industry.

The sanctions are part of the expanded campaign by the United States, Britain and Canada to discourage foreign companies from investing in Iran's oil and gas industry. The three countries argue that the petrochemical profits help fund Iran's nuclear ambitions.

China has repeatedly opposed the U.S. sanctions, insisting they will not help get Iran to abandon its nuclear efforts. Beijing also has sought to separate its trade relations with Tehran from nuclear issues, saying the two are not related.

Tehran denies it is seeking to build nuclear weapons. It says its nuclear program is intended for peaceful purposes.

Under the newly imposed sanctions, the three energy companies are barred from receiving U.S. export licenses, financing from the U.S. Export Import bank, and from getting loans larger than $10 million from U.S. financial institutions.