US Central Command said a short time later that its forces began a third round of strikes against Iran.
"The United States is imposing a heavy cost by continuing to degrade Iran's ability to attack civilian mariners and commercial ships freely transiting the strait," the military said.
A Cyprus-flagged container ship struck by Iran suffered "significant engineroom damage" and a civilian crew member is missing, US Central Command said on Saturday.
Senior US officials had previously said in Washington that negotiations to further cement last month's deal to end the war will be unable to progress without the strait being secure - and had even said they expected Iran to offer public statements to that effect.
Instead, the Revolutionary Guards Corps stated in an online post on Saturday that Iran launched warning shots at a "violating ship".
Iran further reported that the strait would now remain closed until further notice.
Those announcements followed Iran and Oman's foreign ministers meeting on Saturday to discuss the strait that lies between them, after days of Iranian attacks on ships and US retaliation that dealt a blow to the interim deal to end the war.
Iran's new supreme leader, still unseen since the war began, also vowed in his first statement since the funeral of his father, Ayatollah Ali Khamenei that Iranians would avenge his killing in the war's opening strikes on February 28.
Such revenge "is the will of our nation and must certainly be carried out," Supreme Leader Mojtaba Khamenei said in a statement carried on state television, hours after President Donald Trump threatened more missile attacks.
Oman said it and Iran agreed to keep talking about the Strait of Hormuz "at the technical and political levels," a day after the United States called on Iran to publicly say the crucial waterway is open and ships won't be attacked.
Iranian Foreign Minister Abbas Araghchi said he met with his counterpart in Oman to discuss "appropriate mechanisms for ensuring the safe passage of ships."
The world for decades has considered the strait an international waterway. Iran has insisted that the strait now remain under its control and that it be allowed to charge ships moving through it, a stance it took after the war began. The US urges mariners to transit on a southern route through Oman's territorial waters.
About a fifth of all traded oil and natural gas passed through the strait before the war began. Iran's grip on it during the war led to a global energy crisis, though oil prices have sharply dropped since wartime highs of $US120 ($A172) a barrel.
Iran's top diplomat also accused the US of violating the interim deal by ending waivers allowing Iran to sell crude oil on the open market in US dollars. Washington ended them in response to the attacks on ships in the strait.
"Reality check: There can only be mutual compliance," Araghchi wrote on X.
A thousand "missiles are Locked and Loaded and aimed at the Islamic Republic of Iran, with thousands more to immediately follow, should the Iranian Government act on its threat," Trump wrote on social media overnight.
He said he was responding to threats "to assassinate, or attempt to assassinate" him. During Khamenei's funeral, mourners held posters or banners calling for Trump to be killed along with Israeli Prime Minister Benjamin Netanyahu.
Trump has declared the ceasefire over but said the US would continue negotiations.
US officials, speaking Friday on condition of anonymity about the current situation with Iran, said the resumption of strikes in recent days came after what they described as a rogue faction of Iranian hard-liners tried to sabotage the ceasefire.
Iran has insisted its theocracy is unified under the new supreme leader.
After the US wrapped up its latest strikes on Thursday, more attacks reportedly hit Iran, raising questions about who else may be targeting the Islamic Republic.
Israel didn't claim them, meaning the Gulf Arab states may have launched them, likely as a means to deter Iran from attacking them again. Iran on Thursday retaliated for US strikes by targeting Bahrain, Jordan, Kuwait and Qatar.
The strikes in Iran over two days killed at least 17 people and wounded 115 others, Iranian Health Ministry spokesperson Hossein Kermanpour said.
