Central Command said in a social media post the US military launched strikes on Iran's military surveillance capabilities, communications systems and air defence sites.
"US Marine Corps, Air Force, and Navy assets fired precision munitions on Iranian targets that posed a threat to US forces and international commercial ships transiting regional waters," the command said.
"The strikes are in response to Iran's unwarranted and continued aggression. US forces remain vigilant, lethal, and ready."
In response, Iran's Islamic Revolutionary Guard Corps said they had launched counter-attacks on 18 US military targets at air bases in Kuwait and Bahrain, and Bahrain's interior ministry said sirens were sounded.
Iran's top joint military command warned it would fire on any vessel attempting to pass through the the Strait of Hormuz, which has been largely closed for months.
Iranian media reported two ships were fired upon.Central Command denied the strait was closed, saying commercial ships were still transiting the strait despite Iran's threats.
Earlier, President Donald Trump said the US would attack Iran despite a ceasefire, in place since April, and his efforts to reach a deal to end the war.
"We hit them hard yesterday, and we're going to hit them again hard today," Trump had told reporters.
The US attacks are the latest development in an escalating exchange of strikes that threaten to reignite a full-scale war, which was paused in early April when the two sides agreed to a fragile ceasefire.
Speaking to reporters after being briefed at Central Command headquarters in Florida, US Defence Secretary Pete Hegseth said Iran had been given a chance to make a deal but had not taken it.
"As President Trump said, they've been tap, tap, tapping," he said, referring to Iran.
"You can see when someone's trying to tap, tap, tap on a deal, instead they're going to have tap, tap, tap bombs dropping on key facilities in Iran from the United States of America."
Hegseth accused Iran of "choosing to play games" and added: "If we need to negotiate with bombs, we'll negotiate with bombs, and we're very good at it."
The strikes come a day after the US hit Iran following the crash of an Army helicopter near the Strait of Hormuz that Trump blamed on the Islamic Republic.
Earlier Trump revealed the United States has been taking oil out of Iran.
"I'm just announcing today for the first time, but we've been taking out millions of barrels of oil, millions of barrels every night," Trump said, adding that Iran "just figured it out".
"Millions of barrels of oil has come out, and that's why it's at $US85-90 a barrel, instead of $US250," Trump said, sharing no other details about these operations.
He later said more than 100 million barrels of crude had passed through the Straits of Hormuz as part of what he called a secret US mission to support oil tankers.
"More than 200 Commercial Ships have safely travelled through the Strait," he said in a post on Truth Social.
Trump said the United States is still looking to make a deal.
"We want a deal that is meaningful, we want a deal that works," Trump added about the negotiations with Iran.
Trump said Iran has already agreed to not obtaining a nuclear weapon but the agreement still needs to be signed.
Meanwhile, the United Nations nuclear watchdog's 35-country Board of Governors passed a US-backed resolution telling Iran to declare its remaining enriched uranium stocks and let inspectors verify them.
Iran still has not informed the International Atomic Energy Agency of the fate of that material, or let IAEA inspectors return to the bombed sites to check.
with AP and DPA
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