Iranian Foreign Minister Abbas Araqchi announced on social media the strait, a slender chokepoint in global energy trade, was open for all commercial vessels for the remainder of the US-brokered 10-day truce between Israel and Lebanon.
US President Donald Trump, who with Israel launched the war on Iran on February 28, told supporters at a rally in Arizona that Araqchi's announcement marked "a great and brilliant day for the world".
But subsequent statements and clarifications from both sides left uncertainty over how quickly shipping might return to normal, and some vessels could be observed making unsuccessful attempts to cross the strait on Friday, local time, before turning back.
Trump said a US blockade of ships sailing to Iranian ports, announced after talks with Iran last weekend ended without agreement, would remain until "our transaction with Iran is 100 per cent complete".
Iran responded sharply, with its parliament speaker and senior negotiator Mohammad Baqer Qalibaf saying in a social media post that the strait, which until recently carried about a fifth of the world's oil trade, "will not remain open" if the US blockade continues.
He also said Trump had made multiple false claims about the peace talks on Friday.
Iran has said all ships must co-ordinate with the Islamic Revolutionary Guard Corps, which was not the case before the war.
The Defence Ministry said in a statement quoted by state television that military vessels and ships linked to "hostile forces", the US and Israel, were still not permitted to pass.
Vessel traffic data showed a group of about 20 ships, including container ships, bulk carriers, and tankers, moving through the Gulf toward the Strait of Hormuz on Friday evening, but most ended up turning back, although it was not clear why.
The group included three container ships operated by French shipping group CMA CGM, which declined to comment.
It was the largest group of vessels to attempt the transit since the start of the war.
Trump told Reuters the US would remove Iran's stockpiles of enriched uranium. Foreign Ministry spokesperson Esmaeil Baghaei told state TV the material would not be transferred anywhere.
Separately, a senior Iranian official said Iran hoped a preliminary agreement could be reached in the coming days that could extend a ceasefire that is due to expire next week.
That could buy more time for negotiations on lifting sanctions on Iran and securing compensation for war damages, the official said.
Oil prices fell about 10 per cent, and global stocks jumped on news that marine traffic might flow through the strait again.
Shipping companies cautiously welcomed Iran's announcement but said they would require clarifications, including about the risk of mines, before vessels move through the entry point to the Gulf.
The US navy warned seafarers the mine threat in parts of the waterway was not fully understood and said they should consider avoiding the area.
After a video conference on Friday, more than a dozen nations said they were willing to join an international mission to protect shipping in the strait when conditions permit, Britain said.
Trump told Reuters there could probably be more peace talks this weekend. Some diplomats said that was unlikely given the logistics of gathering in the Pakistani capital Islamabad, where the talks are expected to take place.
A Pakistani source involved in mediation efforts said an upcoming meeting could result in an initial memorandum of understanding, followed by a comprehensive peace agreement within 60 days.
