Iran's military declared the Strait of Hormuz closed again on Saturday, prompting ships to abandon attempts to transit the waterway and President Donald Trump to warn Tehran against trying to "blackmail" the United States.

On Friday, Tehran had declared the strait, which usually carries a fifth of the world's oil and liquefied natural gas, open after a ceasefire was agreed in Israel's war with Iran's ally Hezbollah in Lebanon.

This prompted elation in global markets and sent oil prices plunging but, with Trump insisting that a US naval blockade of Iranian ports would continue until a deal to end the wider Middle East war was concluded, Tehran threatened to shutter the strait once more.

Late on Saturday morning, Iran's central military command said that, in response to the US blockade, "control of the Strait of Hormuz has returned to its previous status" and "is under strict management and control of the armed forces".

Trump had previously said the two sides were "very close" to striking a deal, but following Iran's announcement on Saturday, he insisted Tehran "can't blackmail us", even as he maintained communications were ongoing.

"We'll be talking about Iran later. We have very good conversations going on," he added, accusing Tehran of getting "a little cute" with its recent moves.

Iran's top national security body, meanwhile, said that during a visit by the military chief of mediator Pakistan that concluded Saturday, "new proposals have been put forward by the Americans, which the Islamic Republic of Iran is currently reviewing and has not yet responded to".

It went on to say, however, that Iran's negotiating delegation would not offer "even the slightest compromise, retreat or leniency" in talks with Washington.

Ships threatened

A handful of oil and gas tankers crossed the Strait of Hormuz early Saturday during the brief reopening, tracking data showed, but others retreated and tracking platforms showed hardly any vessels crossing the waterway by the late afternoon.

A UK maritime security agency said Iran's Revolutionary Guards (IRGC) fired at one tanker, while security intelligence firm Vanguard Tech reported the force had threatened to "destroy" an empty cruise ship that was fleeing the Gulf.

In a third incident, the UKMTO said that it received a report of a vessel in the same area "being hit by an unknown projectile which caused damage" to shipping containers but no fire.

Speaking at a diplomatic forum in Turkey, Iranian deputy foreign minister Saeed Khatibzadeh said the "Americans cannot impose their will over Iran" with a siege, and suggested the blockade was a violation of the ceasefire that merited "repercussions".

Meanwhile, in a written message, Iran's supreme leader Mojtaba Khamenei, who has yet to be seen since taking power, said Iran's navy "stands ready" to defeat the United States.

Four days left

There are just four days remaining before the end of the two-week ceasefire in the US and Israel's war on Iran, launched by Washington and its ally on February 28.

Nevertheless, Trump has appeared convinced that a deal could be finished shortly, and has made a series of social media posts praising talks mediator Pakistan.

Egypt, which has also been involved in diplomatic efforts, appeared similarly upbeat on Saturday, with Foreign Minister Badr Abdelatty saying Cairo and Islamabad hoped to secure a final agreement "in the coming days". 

He was speaking at the same event in Antalya as Khatibzadeh, who insisted no date had been set for the next round of talks, though Tehran was "very much committed to diplomacy".

Islamabad has emerged as the lead mediator during the conflict, hosting a marathon first round of talks last weekend, with a second expected in the Pakistani capital this coming week.

'Lots of excavators'

The Middle East war began with a massive wave of US-Israeli surprise attacks on Iran, despite Washington and Tehran being engaged in negotiations at the time.

The conflict rapidly spread across the region, with Iran targeting US interests in the Gulf and Hezbollah dragging Lebanon into the conflict by launching rockets at Israel. 

In a sign that the two-week ceasefire remained stable, Iran's civil aviation agency declared its airspace was open again, with international flights able to transit Iran via the east of the country.

Nevertheless, two major sticking points in the peace talks -- Iran's stockpile of near-weapons-grade enriched uranium and the future of the Strait of Hormuz -- appeared up in the air.

Trump declared on Friday that Iran had agreed to hand over its 440 or so kilogrammes of enriched uranium, saying: "We're going to get it by going in with Iran, with lots of excavators."

But Iran's foreign ministry had said just hours before that its stockpile, thought to be buried deep under rubble by US bombing in last June's 12-day war, was "not going to be transferred anywhere", and that surrendering it "to the US has never been raised in negotiations".