The United States has struck bridges in Iran and the Islamic Republic responded by hitting a power and desalination plant in Kuwait as the two sides risked further escalation by expanding their targets to include infrastructure.
At sea, where the renewed conflict has again cut off energy supplies from the Gulf, US Marines boarded a tanker near the Strait of Hormuz.
Armed men seized another vessel off Yemen, raising concern over security in the Middle East's other big choke point for oil shipments at the mouth of the Red Sea.
The Iraninan Tasnim news agency reported late on Friday that the Revolutionary Guards navy had "targeted" a Thai-flagged ship trying to get through the Strait of Hormuz.
It did not give further details.
The US and Iran have been testing the limits of escalation since their ceasefire agreement collapsed last week, raising the prospect of a return to all-out war.
After reports of the escalation emerged on Friday, benchmark Brent crude oil prices climbed three per cent and were on track for a third consecutive weekly gain.
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.
Such moves risk provoking Iran to escalate in turn by hitting the vital infrastructure of vulnerable neighbouring Arab countries, or having its allies in Yemen further disrupt global energy supplies by attacking shipping from the Red Sea.
United Nations Secretary-General Antonio Guterres was concerned about the escalation, particularly over “attacks on civilian infrastructure in Iran and across the region," his spokesperson said.
In the latest strikes, the US military's Central Command included "military logistics infrastructure" in the list of targets it said it had hit, the first time it has mentioned infrastructure in more than a week.
Iranian state media said at least five bridges were 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.
Videos verified by Reuters showed rubble, broken railings and a damaged vehicle on a smashed bridge in Bandar Khamir.
One clip showed a fire.
Iranian Foreign Minister Abbas Araqchi said that three villagers were killed while crossing the Bandar Khamir Bridge, adding that Iran would not allow their blood "to be in vain".
Reuters was unable to confirm details in other reports, which described deadly attacks including one that killed a woman and wounded her child in the port of Bandar Abbas.
Iran announced attacks on Gulf countries that host US air bases, including Bahrain, Qatar and Kuwait.
Authorities in Kuwait said one of the country's power generation and water desalination stations had been hit in an Iranian attack, causing damage, a fire and the disruption of a large number of electricity generation units.
Firefighters brought the blaze under control while technical teams began assessing the damage, securing the station and working to restore power generation as soon as possible, the Ministry of Electricity, Water and Renewable Energy said.
The rich Arab Gulf countries depend on plants that produce electricity and remove salt from seawater to make their desert cities habitable.
When Iran hit a Kuwaiti desalination plant on March 30, it was seen as a major escalation that helped push the United States to declare the war's first ceasefire a week later.
Iran said it also struck a US 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 US special forces base in Tanf, which the US said its forces vacated earlier this year.
A Syrian military source said the strike hit near the base and caused no damage or casualties.
CENTCOM said no US troops were killed or captured.
