The U.S. has advised China that it could be penalized if its banks do business with Iranian financial institutions with ties to Iran’s nuclear program.

U.S. Treasury envoy David Cohen said Wednesday that China’s four biggest state banks could be restricted from some foreign financial systems under United Nations sanctions imposed on Iran for its controversial nuclear program.

He commented in Beijing during a visit that included talks with Chinese officials in Beijing and Hong Kong. During the talks, he discussed ways to prevent Tehran from getting access to global financial markets to fund its nuclear program.

Cohen also says he warned Chinese officials against doing business with the Islamic Republic of Iran Shipping Lines, a bulk carrier blacklisted by the U.S. for its alleged ties to Tehran’s nuclear program.

The U.N. Security Council has imposed four sets of sanctions on Tehran for its uranium enrichment activities and lack of cooperation with international inspectors. Iran says its nuclear program is devoted only to the generation of energy and has peaceful aims.