US forces struck approximately 90 Iranian military targets, according to CENTCOM, the US military's Middle East command.
They included air defence systems, coastal surveillance assets, missile and drone storage sites, naval capabilities, and military logistics infrastructure along Iran's coastline, CENTCOM said in a post on social media platform X.
Iran responded by targeting Bahrain, Kuwait and Qatar in crossfire. Sirens sounded at least twice in Bahrain, home to the US Navy's 5th Fleet headquarters.
There was no immediate word of damage in the three Gulf Arab countries. Kuwait's military said it was actively intercepting incoming drones and missiles. Iran's Revolutionary Guard claimed attacks on Bahrain and Kuwait.
The latest round of US attacks, which the United States said was launched in response to Tuesday's assault on three cargo ships transiting the strait, rattled several cities along Iran's southern coast and left some areas without power.
"US Central Command forces have started conducting additional strikes against Iran to further degrade their ability to threaten freedom of navigation in the Strait of Hormuz," CENTCOM wrote on X earlier.
"The United States is holding Iran accountable for recent unjustified aggression against commercial shipping and civilian crews freely navigating a vital international waterway."
The strikes against Iran will be greater in number than the ones carried out a day earlier, a US official told Reuters, speaking on condition of anonymity.
Iranian state media reported explosions in several locations, including Bushehr, home to Iran's nuclear power plant complex, and the southern port cities of Chabahar, Konarak, Bandar Abbas and Sirik.
In Iranshahr, authorities said a strike had killed a firefighter at an airport.
For the first time since April, it also appeared the US strikes targeted Iranian bridges.
State media reported a strike on a railway bridge in Iran's northeastern Golestan province, and the Revolutionary Guard said two bridges had been attacked on the route to Mashhad, where officials plan to bury the late Ayatollah Ali Khamenei on Thursday.
"This is in retribution for yesterday's bombing of ships by Iran. If it happens again, it will get much worse!" Trump wrote on his Truth Social platform, posting several videos of what he said were explosions in Iran.
Nournews, affiliated with Iran's top security body, cited a military source as saying Tehran would launch a "massive attack" on US bases in the region, a threat echoed by a senior adviser to Supreme Leader Ayatollah Mojtaba Khamenei.
"The aggressor enemy and its accomplices will be severely punished," Mohsen Rezaei wrote on X.
The latest escalation dented hopes of turning a memorandum of understanding signed on June 17 into a permanent peace deal to end the war, which began with US-Israeli airstrikes on Iran on February 28.
Asked before a NATO summit in Turkey whether the memorandum of understanding was over, Trump said: "It's a very interesting question. To me, I think it's over. I don't want to deal with them.
"If we make a deal with Iran I'm not sure that will stick," Trump later said.
"I found them to be very dishonourable people."
But Trump, who has repeatedly threatened to escalate military action before backing off, said he did not expect a return to full-fledged war, and it was not clear whether the negotiations on reaching a permanent deal would continue.
"Anything that happens is going to be over very quickly ... and will only make it safer, including for oil," he said.
with AP
