LONDON, July 13 (Reuters) - The U.N.'s shipping agency said on Monday that it opposes fees on ships passing through maritime waterways but would await more details after U.S. President Donald Trump said he would reinstate a naval blockade on Iran and charge 20% on all cargo shipped through the Strait of Hormuz.

Trump said in a Truth Social post the process would begin immediately, but did not elaborate.

"We are aware of the post and awaiting more details," a spokesperson with the U.N.'s International Maritime Organization said.

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

Shipping industry officials expressed concern at the latest development, adding that such a move in their assessment would violate international law.

"How is this going to make it safer to sail through and what guarantees will this give?" one official, who declined to be identified, said of Trump's post.

(Reporting by Jonathan Saul; Editing by Sharon Singleton and Susan Fenton)