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.