US President Donald Trump said he was calling off strikes on Iran on Thursday and flagged the signing of a possible deal with Tehran after top-level talks.

"Based on the fact that discussions with the Islamic Republic of Iran have been brought to the highest level of Iranian leadership and approved, I have... cancelled the scheduled strikes and bombings against Iran this evening," Trump said on his Truth Social network.

Trump said “final points” had been approved by all parties involved but gave no further details about what had been agreed.

"Time and place of the signing to be announced shortly," added Trump.

However, he also said that the US naval blockade of Iranian ports, which began in mid-April, would remain in force until the “transaction is finalised.”

The social media message marks a shift in tone from a message posted earlier on Thursday in which Trump vowed fresh strikes on, adding that US forces would soon seize the country's key oil infrastructure.

The US military will hit Iran "VERY HARD TONIGHT," Trump posted earlier in the day.

"At some point in the not too distant future, we will be taking Kharg Island, and other oil infrastructure points, and assume total control of their Oil and Gas Markets, much like we have with Venezuela," he added.

Kharg Island is central to Iran's oil exports and any attack on its energy infrastructure could also have serious consequences for global oil markets that have already been pushed to the brink.

About 90% of Iran’s crude oil exports leave the country via the island, much of it destined for China and other Asian markets.

Located in the northern Persian Gulf, Kharg Island is relatively small — about 8 kilometres long and 4–5 kilometres wide — but it hosts extensive infrastructure, including storage tanks, pipelines and offshore loading terminals.

Pipelines from some of Iran’s largest oil fields converge here before crude is loaded onto tankers.

Oil exports remain one of Iran’s main sources of government revenue, making Kharg Island a critical economic asset.

Risk of full-scale war

Trump’s social media post comes after the US and Iran traded strikes for a third day, pushing the Middle East closer to the resumption of a full-scale war.

The American attack, which lasted into Thursday morning in Iran, appeared more intense and wider than the day before.

Iran released little information on the extent of the damage and said it fired back at Kuwait, Bahrain and Jordan, as it had a day before.

The US military continued to enforce its blockade of Iranian ports, saying on Thursday it fired missiles to disable a tanker attempting to transport Iranian oil. An Indian official said a US strike on a different merchant ship earlier this week killed three Indian sailors.

The fresh exchange of fire came as efforts to negotiate an end to the war appeared stalled, with Trump warning that Tehran would “pay the price” for the lack of progress in negotiations.

Iran’s Foreign Ministry said in a statement on Thursday that the US attacks had “effectively rendered the ceasefire...meaningless,” without saying it was abandoning it.

Central to the negotiations is Iran’s stranglehold on the Strait of Hormuz, which has disrupted global energy supplies, driven up fuel prices and made food and other basics more expensive beyond the region.

Iran announced on Thursday that the strait was closed but it was unclear what that meant since it has severely restricted traffic through the waterway since early in the war and only a trickle of ships have passed.

The US military's Central Command disputed the claim and Trump said on Wednesday that the US has undertaken a secret mission in recent weeks to sneak ships through the passage.

The two sides also remain at odds over Iran's nuclear programme, which Tehran insists is peaceful but which the US and Israel fear could be used to build an atomic weapon due to its stockpile of highly enriched uranium.

The US and Israel said a major reason they went to war on 28 February was to ensure that Iran would never be able to do that.

Additional sources • AP, AFP