US President Donald Trump says a ceasefire is still in place between the US and Iran after both sides exchanged fire late on Thursday night. It was unclear who fired first.
Iran's top military command alleged the US had targeted an Iranian oil tanker and another vessel approaching the Strait of Hormuz and carried out "aerial attacks" on several coastal areas, BBC reports.
The US, meanwhile, said it responded to Iranian attacks on US Navy guided-missile destroyers in the Strait with "self-defence strikes". Trump said Iran "trifled with us today".
The flare-up comes a day after Iran's foreign ministry had said it was considering a US proposal to end the war.
In a further sign of the fragility of the ceasefire, early on Friday the United Arab Emirates' Ministry of Defence said its air defences were "actively engaging" with an missile and drone attack from Iran.
Iranian state media initially reported "explosions" in the Strait of Hormuz, describing them as an "exchange of fire" with the "enemy". Meanwhile, local media reported explosions were heard in Tehran.
Not long after, a statement from Iran's top military command said US "aerial attacks" struck the coasts of Bandar Khamir, Sirik and Qeshm Island.
It said it responded immediately by attacking US military vessels, inflicting "significant damage", and accused the US of a "violation of the ceasefire".
US Central Command (Centcom) described the Iranian attacks as "unprovoked", saying Iranian forces launched "multiple missiles, drones and small boats" as US Navy guided-missile destroyers transited through the strait.
Centcom said it had "eliminated inbound threats and targeted Iranian military facilities responsible for attacking US forces, including missile and drone launch sites; command and control locations; and intelligence, surveillance and reconnaissance nodes."
"Centcom does not seek escalation but remains positioned and ready to protect American forces," the statement continued.
Posting on Truth Social, Trump said that the US had destroyed multiple small boats that "dropped ever so beautifully down to the Ocean, very much like a butterfly dropping to its grave!"
"Great damage" was "done to the Iranian attackers," he continued.
The US president also reiterated a warning over the peace deal: "Just like we knocked them out again today, we'll knock them out a lot harder, and a lot more violently, in the future, if they don't get their Deal signed, FAST!"
An Israeli source told the BBC there was "no Israeli involvement" in the latest attacks.