Oil tankers and cargo ships line up in the Strait of Hormuz as seen from Khor Fakkan, United Arab Emirates on March 11. (AP Photo/Altaf Qadri, file)

CAIRO (AP) — U.S. President Donald Trump said Monday that the American military had begun a blockade of Iranian ports as part of his effort to force Tehran to open the Strait of Hormuz and accept a deal to end the war that has raged for more than six weeks.

Iran responded with threats on all ports in the Persian Gulf and the Gulf of Oman, taking aim at U.S.-allied countries.

That set the stage for an extraordinary showdown that posed serious risks for the global economy and raised the specter that the ceasefire could collapse and the war could resume. Talks aimed at permanently ending the conflict — which began Feb. 28 with U.S. and Israeli strikes on Iran — failed to reach an agreement this past weekend. There has been no word on whether negotiations will resume.

Trump said the blockade started at 10 a.m. EDT Monday.

“We can’t let a country blackmail or extort the world because that’s what they’re doing,” Trump said of Iran.

Speaking outside the Oval Office, the president suggested the U.S. is still willing to engage with Iran.

“I can tell you that we’ve been called by the other side,” Trump said.

He added: “We’ve been called this morning by the right people, the appropriate people, and they want to work a deal.”

Trump did not say who called or what was discussed.

A notice to mariners by the U.K. Maritime Trade Operations agency said the blockade restricted “the entirety of the Iranian coastline, including ports and energy infrastructure.”

It said transit through the strait “to or from non-Iranian destinations is not reported to be impeded,” though ships “may encounter military presence.”

At least two tankers approaching the strait Monday turned around soon after the U.S. blockade began, vessel tracker MarineTraffic said in a post on X.

Iran’s effective closure of the strait, through which 20% of traded oil passes in peacetime, has sent oil prices skyrocketing, pushing up the cost of gasoline, food and other basic goods far beyond the Middle East. Tehran has allowed some ships perceived as friendly to pass while charging considerable fees, leading to accusations it is holding the global economy hostage.

Some analysts are doubtful that the U.S. can restore normal shipping through force alone. And it’s not clear how the blockade will work or what the dangers might be to U.S. forces.

The question is essentially who can endure the most pain: Could a blockade make Iran’s economic situation untenable and force it to concede? Or will it drive global oil and other prices so high that Trump is forced to back down?

The U.S. military’s Central Command announced that the blockade would be enforced “against vessels of all nations entering or departing Iranian ports and coastal areas” on the Persian Gulf and Gulf of Oman.

CENTCOM’s decision to allow ships traveling between non-Iranian ports to transit the strait was a step down from Trump’s earlier threat to blockade the waterway.

In a social media message posted shortly after the blockade was due to begin, Trump said Iran’s navy had been “completely obliterated” but still had “fast attack ships.” Trump warned that “if any of these ships come anywhere close to our BLOCKADE, they will be immediately ELIMINATED.”

Threats from Iran

“Security in the Persian Gulf and the Sea of Oman is either for everyone or for NO ONE,” the Islamic Republic of Iran Broadcasting reported Monday. “NO PORT in the region will be safe,” read a statement from the Iranian military.

The threats halted the limited ship traffic that resumed in the strait since the ceasefire, according to a report from Lloyd’s List Intelligence. Marine trackers say over 40 commercial ships have crossed since the start of the ceasefire last week, down from 100 or more vessel passages per day before the war.

The blockade is intended to pile pressure on Iran, which has exported millions of barrels of oil since the war began, much of it likely carried by so-called dark transits that evade Western sanctions and oversight.

But the effects will be felt far beyond Iran. The price of Brent crude oil, the international standard, hovered Monday just under $100 per barrel. It cost roughly $70 per barrel before the war.

British Prime Minister Keir Starmer and French President Emmanuel Macron said they plan a summit this week seeking to end the conflict and unblock the strait. On X, Macron said the conference will include nations prepared to support a “strictly defensive” mission that is “separate from the warring parties.”

U.S. military officials have offered few details about how the blockade will actually work.

The U.S. Navy has 16 warships, including the aircraft carrier USS Abraham Lincoln, in the Middle East, a defense official said. A second defense official said no American warships are in the Persian Gulf, which forms most of Iran’s coastline. Both spoke on condition of anonymity to discuss sensitive military operations.

Under international law, the blockade must be impartially enforced. Legal experts will also be watching to see if the U.S. allows humanitarian aid to reach Iran.

“How it is carried out will determine whether it is lawful or not,” said Todd Huntley, a retired Navy captain and director of Georgetown University’s national security law program.