The US military said it launched "self-defence" strikes on Iran overnight, and shot down ballistic missiles and drones fired at ships and Gulf countries.
The strikes on Qeshm Island, in the Strait of Hormuz, were "in response to attempted attacks by Iran across the Middle East", US Central Command (Centcom) said.
Iran said it had attacked US bases and helicopters in a "regional country" using missiles and drones in retaliation. Centcom said Tehran fired two missiles at Kuwait and three at Bahrain, all of which broke apart or were intercepted.
Kuwait's army later said Iranian drones had hit its international airport, causing "significant" building damage and injuries to a number of people.
Air traffic was suspended on Wednesday morning. Ministry of defence spokesman Brigadier General Saud Abdulaziz Al-Otaibi described the attack as "criminal Iranian aggression". He said the wounded had received medical care.
The latest attacks come amid stalled ceasefire negotiations between the US and Iran, after talks on a deal to end the months-long war failed to advance over the weekend.
Centcom said the strike on Qeshm Island had targeted an Iranian military ground control station, and that the US military also shot down three attack drones that had been launched by Iran toward "civilian mariners that were rightfully transiting regional waters".
The IRGC said "disrupting the security of the Strait of Hormuz will carry a heavy price for the aggressive US military".
Centcom also said Iran had launched "several" ballistic missiles towards countries in the region. "Two Iranian missiles fired at Kuwait fell short or broke apart en route, and three missiles launched at Bahrain were immediately intercepted by US and Bahrain air defence forces."
Iran has repeatedly attacked targets in Bahrain and Kuwait, where US military bases are located.
Earlier, Centcom said it had struck and disabled an unladen oil tanker that was sailing towards Iran, as part of Washington's naval blockade on the Strait of Hormuz, which began on 13 April.
A US aircraft fired a Hellfire missile into the engine room of the Botswana-flagged M/T vessel, after its crew "ignored repeated warnings", it said.
Centcom also released footage purportedly showing the moment the tanker was hit on Tuesday.
The escalation comes after US President Donald Trump this week told his critics to "sit back and relax", saying that Iran "really wants to make a deal, and it will be a good one for the USA".
US media earlier reported that Trump had requested edits to the terms of a potential peace deal, after meeting with senior aides to discuss extending the framework of a ceasefire.
The changes related to the Strait of Hormuz and the removal of highly enriched uranium from Iran, the BBC's US news partner CBS News reported - as well as a framework to reopen negotiations on Iran's nuclear programme.
On Monday, Iran's foreign ministry spokesman Esmail Baghaei denied this had been on the table, adding that Washington was "constantly changing its views and putting forward new or contradictory demands".
In its recent statement, Centcom said US forces "enforced blockade measures against Botswana-flagged M/T Lexie as it transited international waters toward Kharg Island".
It said the ship's crew had failed "to comply with directions from US forces multiple times over a 24-hour period".
Overall, six commercial vessels have been disabled and another 122 redirected since the blockade went into force, Centcom said.
The BBC has contacted Botswana's government for comment.
The latest skirmish comes as US Secretary of State Marco Rubio appeared publicly before Congress for the first time since the start of the war.
The top diplomat testified that US negotiators had not offered Iran sanctions relief in exchange for re-opening the strait.
"Right now, everything that's been discussed with them is that … any sanctions relief is condition-based, which means it has to be in return for the reason why those sanctions were put in place in the first place, which is their nuclear programme," he said.
"The war is over," he said in another tense exchange with a senator, as lawmakers on the committee questioned the US strategy for ending the conflict.
