The United States escalated its renewed bombing campaign on Iran on Friday by hitting bridges and an airport, and Tehran responded with strikes on US bases across the West Asia.

In the contested Strait of Hormuz, where the renewed conflict has again cut off global energy supplies from the West Asia, US Marines boarded a tanker and another ship was reported to have been hit by a projectile.

The warring sides have been testing the limits of escalation since their ceasefire agreement collapsed last week, raising the prospect of a return to all-out war.

US President Donald Trump has threatened to launch broad-based air strikes on Iran's infrastructure, and has also declined to rule out a ground assault on Iran's coast or islands. US officials have said attacks on southern Iran are designed in part to give Trump options.

But such moves risk provoking Iran to escalate in turn by hitting infrastructure of neighbouring countries, or further disrupt energy supplies by having its allies in Yemen attack shipping from the Red Sea.

The US military's Central Command included "military logistics infrastructure" in the list of targets it said it had struck in its latest attacks on Iran, the first time it has mentioned infrastructure in more than a week.

For now, the attacks appeared to be limited mainly to southern coastal areas that have already been targeted intensively in recent days.

Iranian state media said at least five bridges had been struck in the south. Seven people were reported killed in attacks on bridges in the southern port of Bandar Khamir, where the train station was also hit. An airport was reported hit further east and away from the coast in Iranshahr, in a province bordering Pakistan.

Reuters could not verify the reports, which also described other deadly attacks including one which killed a woman and wounded her child in the port of Bandar Abbas.

Fued over Stait

Iran said it had struck US bases in Kuwait and Bahrain, and a US radar station in Oman. Explosions were also heard in the Qatari capital Doha, where the interior ministry said a child was wounded by shrapnel.

Iran said it fired at Syria, apparently for the first time in the war, targeting what it described as a US special forces base in Tanf.

Syria says US 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 Jul 7, when Iran struck ships in the Strait of Hormuz and the United States responded with air strikes.

The renewed fighting has once again largely halted traffic through the world's most important energy shipping route, sending oil prices LCOc1 surging to around $85 a barrel this week.

Iran has announced the closure of the strait, and Washington has reimposed its own blockade of Iranian ports.

In the latest incidents at sea, the US military said it had boarded the tanker Wen Yao 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.

The British maritime security service UKMTO reported a tanker had been hit by a projectile on Thursday off the coast of Oman.

While both sides have exchanged strikes daily, 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 West Asia 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 worsening the global energy crisis by cutting off another route out for West Asia oil. 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 unpopular war as quickly as possible. But leaving Iran in control of the strait would be a strategic embarrassment in a region where US forces have served as the main security guarantor for generations.

In a televised speech on Thursday night that was 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."