President Trump on Wednesday warned the Gulf state of Oman, a U.S. ally, not to enter into any agreement with Iran to share control of the Strait of Hormuz or else face an American bombing campaign.

Quickly after making the threat, Mr. Trump said he didn’t believe the United States would have to take such an action.

“Oman will behave just like everybody else, or we’ll have to blow them up,” he said during a cabinet meeting, responding to a reporter’s question about the strait. “They understand that. They’ll be fine.”

Iran has discussed partnering with Oman in a system charging fees for vessels passing through the Strait of Hormuz, ignoring the Trump administration’s warnings against demands for payment to pass through the critical international waterway.

Mr. Trump said there would be serious repercussions if the two countries agree to such a deal.

“The strait is going to be open to everybody,” he said. “Nobody’s going to control it. We’re going to watch over it. We’ll watch over it, but nobody’s going to control it.”

Oman’s foreign ministry did not immediately respond to a request for comment.

The United States and Israel attacked Iran on Feb. 28, starting a monthslong war that spread to neighboring countries. In response, Iran closed the strait, a key passageway for global commerce, rocking the financial markets.

Despite talks about a potential deal to quickly reopen the strait, the likelihood of a swift diplomatic breakthrough seemed dim Wednesday after the United States and Iran ratcheted up hostilities this week.