US President Donald Trump has threatened to "knock out every single power plant and every single bridge in Iran" if Tehran does not accept a US-proposed deal, following an incident in the Strait of Hormuz that Washington said involved Iranian vessels firing at French and British ships.

In a post on Truth Social on April 19, Trump said Iran had "decided to fire bullets yesterday in the Strait of Hormuz" and called the incident "a total violation of our ceasefire agreement."

"Many of them were aimed at a French Ship, and a Freighter from the United Kingdom. That wasn't nice, was it?" Trump wrote. He said US representatives would travel to Islamabad for negotiations the following evening.

Trump said the US naval blockade had already closed the strait and that Iran's decision to reopen it to commercial shipping on April 17 was pointless. "They're helping us without knowing, and they are the ones that lose with the closed passage, $500 million dollars a day," he wrote.

The president said many ships were now heading to US ports in Texas, Louisiana and Alaska to load up, "compliments of the IRGC."

"We're offering a very fair and reasonable deal, and I hope they take it because, if they don't, the United States is going to knock out every single power plant, and every single bridge, in Iran. No more Mr. Nice Guy!" Trump said.

The president ended the post by saying it would be his honour to do what should have been done to Iran by other presidents over the past 47 years, adding: "It's time for the Iran killing machine to end!"

Iran has rejected Trump's framing. An informed source told IRGC-linked Tasnim News Agency on April 19 that Tehran would not send a negotiating team to a second round of talks while the US naval blockade remained in place.

The Iranian team has informed Washington through Pakistani mediators that no negotiations will take place as long as Trump's blockade remains in force, the source said. Message exchanges via Islamabad have continued since the breakdown of the first round but Iran has hardened its position.

Iran has not publicly commented on the alleged firing incident in the Strait of Hormuz.

Tehran reopened the strait to commercial shipping on April 17 for the remainder of the two-week ceasefire, under conditions set by the Islamic Revolutionary Guard Corps, including that passage be coordinated with Iranian forces and that vessels not be linked to hostile countries.

France and Britain chaired a meeting on April 18 of around 40 countries on the international role in the strait, though the initiative does not currently include the US or Iran.