Iran said it had struck U.S. bases in Kuwait, Qatar and Bahrain, and a U.S. radar station in Oman. Explosions were heard in the Qatari capital Doha, where the interior ministry said a child was wounded by shrapnel.
Iran also said it fired at Syria, apparently for the first time in the war, targeting what it described as a U.S. special forces base in Tanf. Syria says U.S. forces pulled out of the base there earlier this year. A Syrian military source said the strike hit near the base and caused no damage or casualties.
An interim agreement to end the war has collapsed since July 7, when Iran struck ships in the Strait of Hormuz and the United States responded with air strikes.
The escalation pushed the benchmark price of Brent crude up by another 2% to around $86 LCOc1, its highest level since an interim agreement was reached a month ago to end the war.
Iran has announced the closure of the strait, and Washington has reimposed its own blockade of Iranian ports.
In the latest action at sea, the U.S. military said it had boarded a tanker to enforce the blockade, releasing photos of Marines rappelling down from a helicopter onto the deck where one posed in front of an Iranian flag.
Beyond the Gulf, armed men seized a small chemical tanker off Yemen in the Gulf of Aden, close to the mouth of the Red Sea. One maritime security source said the incident appeared to be related to Somali piracy rather than Iran's Yemeni allies, the Houthis.
While Iran and the United States have exchanged strikes daily since last week, they have so far stopped short of escalating beyond parameters set earlier in the war, when civilian infrastructure and major economic targets were mostly deemed out of bounds because of the threat of retaliation.
Iran has said that it would attack civilian infrastructure across the Middle East if Trump follows through on threats to attack Iran's infrastructure.
It has also signalled that it could prod its Houthi allies in Yemen to close another key strait: the Bab al-Mandeb at the mouth of the Red Sea, potentially cutting off the main alternative route for Middle East oil bypassing the Gulf. Sources have told Reuters Iran has already instructed the Houthis to act if Washington attacks Iran's infrastructure.
The surge in energy prices generated by the conflict has created pressure on Trump to end the war quickly. In a televised speech on Thursday night mainly about election security, Trump said the United States was "winning big in Iran, and you will see the fruits of that labour very, very shortly."