The price of Brent crude oil, the international benchmark, settled above $80 a barrel after President Donald Trump said the U.S. naval blockade of the Strait of Hormuz will be reinstated after a weekend of ongoing strikes by both the U.S. and Iran in the region.

The blockade will start at 4 p.m. Eastern time, shortly before midnight in Iran, U.S. Central Command announced.

Trump added that all non-Iranian ships that pass through the strait must reimburse the U.S. at a rate of 20% for the cargo. He did not specify how that charge would be calculated and the White House didn’t respond to Barron’s request for comment.

But the cease-fire has always been fragile. Most recently, Iran has taken the position that the memo of understanding gave it control of the strait, but the U.S. has pushed back on that. Trump wrote that the U.S. “will be, from this point forward, known as ‘THE GUARDIAN OF THE HORMUZ STRAIT.’”

The news sent oil prices, which were already up more than 3% before the announcement, up another six percentage points to $83.30 a barrel for Brent Crude and $78.14 for West Texas Intermediate. The last time Brent settled above $80 a barrel was more than three weeks ago on June 19. On an intraday basis, it topped $80 last Wednesday, according to FactSet, after the U.S. struck Iran’s main oil terminal in the Persian Gulf, Kharg Island.

Iran’s foreign minister, Abbas Araghchi, wrote on X that “Iran has always been the GUARDIAN of the Strait and will remain so FOREVER.” He added that whoever provides passage through the strait should be compensated, but that “20% is of course too much. We will be fair.”

Meanwhile Kpler’s MarineTraffic data service reported Monday that commercial crossings through the strait had fallen 52% over the weekend compared with the prior weekend, with just 12 sanction crossings observed on Sunday. Before the war began, more than 100 ships crossed the critical waterway each day.

“I think many participants believe that this latest flare up in fighting will be temporary,” RBC Capital Markets head of global commodity strategy Helima Croft told Barron’s. “However we continue to be of the view that we will not get back to pre war transit levels any time soon and that the Middle East shuts will persist and hit countries without viable work arounds the hardest.”

On Sunday, Goldman Sachs analysts noted that the oil price rally in recent days “demonstrates how critical Hormuz flows remain for prices in the short-term.” While there are various workarounds, including land-based pipelines in Saudi Arabia and the United Arab Emirates, they aren’t broad enough to negate the impact of the strait’s closure.

“There are still signs that a bill may be coming due soon,” RBC Markets’ Croft said, if the strait shuts down again.

“More pain could certainly be in store in a prolonged Hormuz paralysis situation, especially given the depleted inventory situation,” she added. The U.S. and other countries relied on their own reserves to help soften the impact of the strait’s closure, which helped keep prices in check. However that has left reserves low.

The one wild card remains China, which sharply pulled back on purchases as the conflict rose, relying instead on their strategic reserves. “You could really make the case that their purchase pullback really was the key to averting a hard landing,” Croft added.

Write to Anita Hamilton at anita.hamilton@barrons.com