President Trump said on Sunday that the United States would launch a new effort to help guide stranded ships out of the Strait of Hormuz, which Iran has effectively closed.

Mr. Trump said the initiative, called Project Freedom, would begin on Monday morning “Middle East time,” after his administration heard from nations seeking help freeing their ships. He warned that any interference in the program would be dealt with “forcefully,” but offered few details about how it would work.

“These are Ships from areas of the World that are not in any way involved with that which is currently taking place in the Middle East,” Mr. Trump said in a lengthy social media post.

Mr. Trump’s announcement was essentially a challenge to Iran, and a bet that it would not want to take the risk of firing the first shots — or laying mines — in a challenge to the U.S. Navy. He said nothing about lifting the American blockade on all shipping in and out of Iranian ports.

If the move works, it could flip the script on the current dual blockades — one run by the Iranians, the other by the United States. But it could also lead to a breach in the current cease-fire if Iran sought to try to intercept shipping or challenge the American effort.

Mr. Trump did not make clear in his post what it meant for the United States to “guide” ships.

But a statement issued by the U.S. Central Command on Sunday evening indicated that the American role would involve coordinating safe traffic among the stranded ships, rather than escorting them.

“U.S. military support to Project Freedom will include guided-missile destroyers, over 100 land and sea-based aircraft, multi-domain unmanned platforms, and 15,000 service members,” Centcom said in the statement, posted on social media.