The latest escalation of attacks and counterattacks launched by the US and Iran has kept the region on edge days after a fragile truce collapsed, raising the spectre of a return to full-scale war although analysts generally view that as less likely.
Hostilities have intensified since Iran said late on Saturday it had closed the Strait of Hormuz.
The ongoing military operations are also keeping ships from transiting the vital artery, which carried about a fifth of global oil and gas shipments before the war.
Brent crude oil, the international benchmark, closed at a one-month high at $US84.95 a barrel on Wednesday.
US Central Command said the military had attacked coastal defence systems and cruise missile storage and launch sites on Iran's Greater Tunb Island, and had completed the wave of strikes within about 90 minutes.
"At 6am today, US Central Command forces began launching a wave of strikes against Iran," the US military said.
"The strikes are designed to further degrade military capabilities Iranian forces have used to attack commercial shipping in the Strait of Hormuz."
In a statement issued hours after the Iranian Mehr news agency reported that US projectiles had hit a location on Iran's Hengam Island in the strait, Iran's top negotiator Mohammad Baqer Qalibaf said Iranian security depended on maintaining what he called "Iranian arrangements" in the strait.
"We are in an essential and existential war with America," Qalibaf said in a statement.
Prior to the war, Iran had not asserted the authority to act as a gatekeeper over the strait.
The war has killed thousands of people and displaced millions, mainly in Iran and Lebanon, where conflict restarted between Israel and Iran-aligned militant group Hezbollah.
US President Donald Trump on Wednesday said that Iran badly wants to
settle with the United States, adding that the US would decide whether or not to take such a step.
Trump said on Tuesday that US negotiators had been in touch with their Iranian counterparts to tell them "you better make a deal".
Three US officials told Reuters that US strikes aimed at forcing open the strait are also targeting Iranian military capabilities the US would want to destroy before executing more complex operations.
Shipping traffic through the Strait of Hormuz has dropped to a fraction of its pre-war activity.
Iran's Islamic Revolutionary Guard Corps said on Wednesday it had struck US military targets in the region, including in Bahrain, Kuwait and Jordan.
It also threatened on Wednesday to shut off more regional energy exports, saying the US "must brace for the closure of all other export corridors that benefit the US and its allies".
An interim ceasefire deal signed last month was meant to lead to further negotiations including on Iran's nuclear program and to a permanent truce but a return to talks has faltered.
"We have no plans for negotiations at the moment and are focused on defence," Tasnim news agency quoted Iranian foreign ministry spokesman Esmaeil Baghaei as saying.
Qalibaf, who is also speaker of Iran's parliament, said that if Iran did not benefit from its memorandum of understanding with the US, "we have no reason to adhere to such an understanding".
Iran never welcomed war but it must always be ready to fight and "stand to the end" to protect national security and Iranian interests, Qalibaf said.
He added Iran must also use "the tools of diplomacy and negotiation," and choosing either negotiation or war as the sole course of action would be a strategic error.
The US says Iran attacked seven commercial ships over the last week, leaving nearly a dozen crew members killed, missing or injured.
Iranian government spokeswoman Fatemeh Mohajerani said at least 30 civilians had been killed in recent days due to the US strikes on southern Iran, state media reported.
