Shipping companies they could face sanctions if they pay Iran fees for safe passage through the Strait of Hormuz, warned the U.S. Office of Foreign Assets Control (OFAC) on May 1.

Hamidreza Haji Bababei, deputy speaker of Iran's Parliament claimed last week that Iran has collected tolls from ships already, and that the first toll revenue had been deposited with the country's Central Bank. No further details were available.

BBC News reports that the OFAC said U.S. persons and companies were generally banned from paying Iranian government entities, and non-U.S. persons may risk exposure to sanctions if they pay.

"Maritime industry participants involved with vessels calling at Iranian ports face significant sanctions risk under multiple sanctions authorities targeting Iran's shipping sector and ports," OFAC said.

The agency warned that non-U.S. persons who pay could also face civil and criminal enforcement liability if payments cause U.S. persons, such as insurers and financial institutions, to violate sanctions.

OFAC said it "will continue to aggressively target Iran's main revenue-generating sectors, in particular its petroleum and petrochemical sectors."

The BBC says U.S. Treasury also announced sanctions on three Iranian foreign currency exchange houses on May 1, saying they have converted oil revenue into more usable currencies.