The Obama administration is putting new pressure on Iran by blacklisting 37 shipping companies in Germany, Malta and Cyprus that the U.S. links to Tehran's nuclear program, the Associated Press reported.

The Treasury Department said Wednesday that all are front companies owned or acting for the Islamic Republic of Iran Shipping Lines, which the U.S. says is used to advance Iran's nuclear and missile programs.

The blacklisting is the latest in a series of actions by the U.N., as well as the U.S. and other countries to punish Iran for its disputed nuclear program. The U.S. measures are also intended to pressure Tehran into agreeing to taking steps to prove that the program is not used to make bombs.