US President Donald Trump said he was sending negotiators to Pakistan on Monday for talks with Iran just days before a ceasefire in the Middle East expires, though Tehran has reportedly yet to decide whether it will participate.

Announcing the fresh diplomatic push, Trump also renewed his threats against Iran's infrastructure, saying in a post on his Truth Social account Sunday that without a deal, the US was "going to knock out every single Power Plant, and every single Bridge, in Iran. NO MORE MR. NICE GUY!"

Iran, however, appeared cool to the prospect of talks, particularly in light of the ongoing US blockade of its ports, according to Iranian media reports.

The Fars and Tasnim news agencies cited anonymous sources as saying Tehran had yet to decide whether it would take part and that "the overall atmosphere cannot be assessed as very positive", adding that lifting the US blockade was a precondition for negotiations.

State-run IRNA, meanwhile, pointed to the blockade and Washington's "unreasonable and unrealistic demands", saying that "in these circumstances, there is no clear prospect of fruitful negotiations".

Iran and the United States, along with Israel, are just three days away from the end of the two-week ceasefire that halted the Middle East war, ignited by surprise US-Israeli strikes on Iran on February 28.

There has so far been only a single, 21-hour negotiating session held in Islamabad on April 11 that ended inconclusively, though groundwork for fresh talks continued afterwards.

"We're offering a very fair and reasonable DEAL, and I hope they take it," Trump said in his post.

Iran's speaker of parliament and senior negotiator Mohammad Bagher Ghalibaf had insisted as recently as Saturday night that the two sides were "still far from the final discussion".

Heightened security

In spite of the uncertainty, security was visibly stepped up in Islamabad on Sunday in anticipation of new talks.

Authorities announced road closures and traffic restrictions across the city, as well as in neighbouring Rawalpindi.

AFP journalists saw armed guards and checkpoints near Islamabad's most secure hotels -- the Marriott and the Serena.

"Citizens are earnestly requested to cooperate with the security agencies," a city official posted on X.

The US president said his negotiators, whom he did not name, would arrive in the Pakistani capital on Monday evening.

As with the last round, the delegation will be led by Vice President JD Vance and include Middle East envoy Steve Witkoff and Trump's son-in-law Jared Kushner, a White House official said.

A major sticking point of negotiations has been Iran's stockpile of near-weapons-grade enriched uranium.

Trump said on Friday that Iran had agreed to hand over its roughly 440 kilogrammes of enriched uranium. "We're going to get it by going in with Iran, with lots of excavators," he said.

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

On Sunday, President Masoud Pezeshkian questioned why Iran should give up its "legal right" to a nuclear programme.

Hormuz closed again

Trump has been under pressure to find an off-ramp to the war since Tehran moved early in the war to choke off the Strait of Hormuz.

The vital waterway is a conduit for a fifth of the world's oil and liquefied natural gas in peacetime, and its closure has hammered the global economy and roiling markets.

Having failed to force it open again, Trump countered with a US naval blockade on Iranian ports in an attempt to cut off Tehran's oil revenues.

Iran briefly reopened the strait on Friday in recognition of an Israel-Hezbollah ceasefire in Lebanon, but closed it again the following day in response to the US maintaining its blockade. 

Iran's Revolutionary Guards warned that any attempt to pass through the strait without permission "will be considered cooperation with the enemy, and the offending vessel will be targeted".

"If America does not lift the blockade, traffic in the Strait of Hormuz will definitely be limited," Ghalibaf said.

Iranian foreign ministry spokesman Esmaeil Baqaei on Sunday said the blockade was "a violation" of the ceasefire and illegal collective punishment of the Iranian people.

A handful of oil and gas tankers had crossed the strait early on Saturday during the brief reopening, but by early Sunday morning tracking data showed the waterway empty of shipping.

The afternoon before, a trio of incidents involving Iranian fire and threats towards commercial vessels demonstrated the danger of any attempted crossing.

In his post Sunday, Trump said of the incidents: "That wasn't nice, was it?"