The United Nations agency responsible for creating and maintaining a regulatory framework for international shipping said Monday it opposes any transit fees for commercial vessels passing through the Strait of Hormuz.

The statement by an International Maritime Organization spokesperson came after President Donald Trump earlier Monday proposed a 20% cargo fee on all commercial vessels passing through the strategic waterway to reimburse the U.S. Navy for providing security and escorts.

"We have always been consistent on our stance on fees — IMO stands firmly against charging fees for passage through straits used for international navigation," the spokesperson told CNBC. "There is no legal basis through which to introduce mandatory tolls simply to transit through a strait."

Nordic American Tankers CEO Herbjorn Hansson told CNBC that Trump's 20% fee is unrealistic, adding that Iran and the U.S. have to agree on how the strait will be administered.

"Iran is suffering, America is suffering, 192 countries outside the Hormuz Strait are suffering," Hansson said.

In June, months after U.S. and Israel launched a campaign targeting Iran's political leadership and military infrastructure, Tehran implemented tolls of up to $2 million per vessel passing through the strait, one of the world's key energy chokepoints.

Tehran assured it would use its "best efforts" to ensure the safe passage of commercial vessels through the strait for 60 days under the memorandum of understanding it signed with the U.S. on June 17. But it has interpreted that to mean the U.S. officially recognizes its right to manage and regulate traffic through the waterway.

The U.S. and its regional partners rejected that interpretation. They argued the agreement required Iran to facilitate only safe transit and explicitly banned it from imposing restrictions by force.

Commercial vessels have instead been using an alternate route hugging Oman's coast under U.S. naval protection.

Iran has repeatedly fired on those vessels and recently declared the strait closed.

The U.S. stated that Iran failed to adhere to the agreement, and Trump declared the MOU "over."

Transit tolls through the strait are against international law, James Kraska, an expert on international maritime law at the U.S. Naval War College, told CNBC.

He said Iran cannot unilaterally impose a change to traffic routes through Hormuz under the Convention on the International Maritime Organization and the International Convention for the Safety of Life at Sea.

He added that Iran has a legal duty to comply with the traditional route through Hormuz, known as the traffic separation scheme, because of its obligations under those treaties.