The US blockade of ships using Iranian ports in the Gulf was due to take effect on Monday evening, turning the six-week-old conflict between the US-Israeli coalition and Iran into a test of economic endurance.

US Central Command (Centcom) made no formal announcement of the start of the blockade but had said it would take effect at 5.30pm Iranian time (3pm UK time) on Monday, and would apply to any ships entering or departing Iranian ports or coastal areas, while ships using non-Iranian ports would not be impeded.

Donald Trump claimed that 34 ships had passed through the strait of Hormuz, the gateway to the Gulf, on Sunday, but there was no supporting evidence for the claim. Speaking to reporters at the White House, the president also claimed: “We’ve been called by the other side,” who he said would “like to make a deal very badly.”

Throughout the conflict, which began with a US-Israeli attack on 28 February, Trump has made frequent claims that Tehran had been in direct contact, desperate for an agreement, but none of those claims has ever been substantiated.

Iran warned that ordinary Americans would pay the cost for Donald Trump’s latest move in the shape of higher petrol prices, and also vowed that if the US went back to bombing, the Tehran regime was ready to retaliate. For his part, Trump said any Iranian attack boats approaching the US flotilla in the region would be “immediately eliminated”.

It appeared on Monday that US naval forces were going to try to enforce the blockade east of the strait of Hormuz, in the Gulf of Oman, beyond easy Iranian missile and drone range. It remained unclear how Centcom intends to stop any oil tanker attempting to break the blockade. A missile attack could cause an environmental disaster, leaving open the possibility that US forces could seek to board and take control of any vessel not obeying US instructions.

UK Maritime Trade Operations issued an advisory to seafarers to “maintain heightened situational awareness” pending updates giving details on how they were expected to navigate through the new conditions in the region.

Trump issued a warning that any Iranian “fast attack ships” would be “immediately eliminated” if they approached US vessels enforcing the blockade, with “the same system of kill” as the US has used to sink nearly 50 small boats in the Caribbean and eastern Pacific, killing at least 168 people who it has claimed without evidence were involved in narco-trafficking.

Trump ordered the blockade following US-Iranian talks in Islamabad that ended after 21 hours without agreement.

The tactic is aimed at strangling the heavily oil-dependent Iranian economy, and forcing Tehran to meet US demands to reopen the Hormuz strait to ships from the ports of Gulf allies, and to accept a complete ban on uranium enrichment.

Miad Maleki, a former US treasury official now at the Foundation for the Defense of Democracies, said on X that the US naval blockade would cost Iran apporoximately $276m a day in lost exports and disrupt $159m a day in imports – representing combined economic damage of $13bn a month.

The Iranian regime has insisted that it still have effective control of the Hormuz strait and can determine which ships will be allowed to pass, and has pointed out that the US blockade will result in higher oil prices, which have already climbed back to above $100 a barrel since the diplomatic breakdown in Islamabad.

Mohammad Bagher Ghalibaf, Iran’s parliamentary speaker who also led his country’s delegation in Islamabad, told Americans in a post on X on Sunday to “enjoy the current pump figures”, taunting Washington with historical US political sensitivity about petrol prices.

“With the so-called ‘blockade’, soon you’ll be nostalgic for $4–$5 gas,” Ghalibaf added. The current average petrol price in the US is $4.13 a gallon, up from $2.98 before the US and Israel launched the war on Iran on 28 February.

The president conceded on Sunday that petrol prices could be the same as they are now or more when the nation votes in congressional elections, telling Fox News they could go “a little bit higher”.

The Iranian embassy in Thailand posted a mock election poster on Monday, emblazoned with the words “Trump: $20.28 a gallon”, under the question: “Are you ready folks?”

On Monday Iran inserted itself into Trump’s continuing spat with Pope Leo XIV over the conflict.

The US president had reacted angrily to the US-born pope’s criticisms of the administration’s use of religious language to justify its war in Iran. Trump called him “weak on crime” and “terrible for foreign policy”, and posted an AI-generated picture of himself as a Christ-like figure tending the sick, an image widely condemned as blasphemous. On Monday, Trump claimed the image (in loose red and white robes and light emanating from his hands) was intended to portray him as “a doctor”.

Iran’s president, Masoud Pezeshkian, spoke out on Monday against what he called the “desecration of Jesus”.

“I condemn the insult to Your Excellency on behalf of the great nation of Iran, and declare that the desecration of Jesus, the prophet of peace and brotherhood, is not acceptable to any free person.”

The pope told reporters on Monday that he has “no intention to debate” with Trump over Iran and added he would “continue to speak out loudly against war, looking to promote peace, promoting dialogue and multilateral relationships among the states to look for just solutions to problems”.

US-Israeli bombardment of Iran has stopped under a two-week Pakistani-brokered ceasefire, which began on Wednesday. Trump has said US forces remained “locked and loaded” and ready to “finish up the little that is left of Iran”.

Iran has also said it is ready to go back to battle. Ebrahim Zolfaghari, an Iranian military spokesperson, said on Monday that if Iranian ports were threatened, “no port in the Persian Gulf and the Sea of Oman will be safe”.

Despite Trump’s claims that other countries would help enforce the US blockade, none has come forward. The UK prime minister, Keir Starmer, was adamant that his country did not support the blockade and that “we are not getting dragged into the war”.

Germany, Spain, Italy, Poland and Greece have all ruled out sending naval forces to support the blockade. France’s president, Emmanuel Macron, has said Paris would organise a conference with the UK and other countries to create a multinational mission to restore navigation in the Hormuz strait but made clear that would come after the conflict.

“This strictly defensive mission, distinct from the belligerents, will be deployed as soon as the situation allows,” Macron said on X.

Ursula ⁠⁠von der Leyen, the president of the European Commission, said that restoration ⁠⁠of freedom of navigation in the strait of ⁠⁠Hormuz is of “paramount” importance.