The US military said "self-defense" strikes had targeted Iranian air defence, ground control stations and surveillance radar sites near the Strait of Hormuz.
US Central Command described the operation as a "proportional response" to Iran's downing of a US Apache helicopter off the coast of Oman and recent attacks on US forces and commercial shipping.
"CENTCOM forces struck Iranian air defense, ground control stations, and surveillance radar sites near the Strait of Hormuz with precision munitions from US Air Force and Navy fighter jets," the post on X said.
"US forces remain vigilant and postured to defend against unjustified Iranian aggression," CENTCOM said.
The strikes began at 5pm on Tuesday (7am Wednesday AEST) and Central Command posted just before 9pm that they had ended.
Iran's state media reported that Qeshm island and the port city of Sirik in the Strait of Hormuz were attacked.
Sounds of explosions were heard in nearby Bandar Abbas, and later in the vicinity of Jask county, near the entrance to the strait, Iranian media reported, citing local sources and residents.
Some US bases in the region were targeted in response to the strikes, Iranian media cited the country's top joint military command, Khatam al-Anbiya, as saying.
Iran's Revolutionary Guards said they attacked the US Fifth Fleet in Bahrain with drones and threatened "more severe responses" if hostilities continued, according to media.
Bahrain's Interior Ministry said a warning siren had been sounded and urged the public to head to safety. Air defences had repelled Iranian attacks, a media adviser to Bahrain's King said soon after in a post on X.
Reuters could not immediately verify the battlefield reports. Oil prices climbed about one per cent in early Asian trade on Wednesday following the escalation in hostilities.
The Apache was brought down by a one-way Iranian attack drone, according to a US official who spoke on condition of anonymity. Two US pilots involved in the helicopter incident were uninjured, according to Trump.
Iran's state media cited a military source as saying that no offensive air military operations had been conducted in the Strait of Hormuz in the previous 24 hours.
Following the initial US strikes, Iran's Foreign Minister Abbas Araqchi posted on X that the country would "leave no attack or threat unanswered".
In an earlier post, he did not directly address the helicopter incident, but said foreign forces in the region risked being involved in accidents or crossfire.
"To reduce risk, best solution is for them to leave," he wrote.
Iran and Israel exchanged airstrikes earlier this week, killing two people in Tehran.
Trump told The Wall Street Journal during a phone call on Tuesday that the helicopter incident "wasn't a big deal" and stressed that "the pilot is fine".
However, the episode could well add further strain to efforts to broker a peace deal to end the wider Middle East war and reopen Hormuz, a vital conduit for petroleum and other commodities.
Trump has repeatedly said Iran and the United States are close to an agreement, though there have been few signs of progress since a tenuous ceasefire took effect in early April.
A US Navy surface drone found and rescued the two crew, the US military said, after the US Army attack helicopter went down in waters near Oman's coast while on patrol at around 3am on Tuesday.
The US military's Central Command gave no reason for the crash. It said the soldiers were rescued after two hours and said they were in stable condition - a more cautious assessment than Trump's description.
