Donald Trump has issued a fresh ultimatum, telling Iran to accept a deal to end the war or face a new wave of US bombing “at a much higher level and intensity than it was before”.

The social media announcement on Wednesday was the latest in a rapid series of dramatic and often contradictory changes in policy and came amid unconfirmed reports of progress in stalled negotiations between Tehran and Washington.

“Assuming Iran agrees to give what has been agreed to, which is perhaps a big assumption, the already legendary Epic Fury will be at an end,” the US president posted on his Truth Social platform, referring to the military operation he launched with Israel against Iran in February.

“If they don’t agree, the bombing starts, and it will be, sadly, at a much higher level and intensity than it was before.”

The Israeli prime minister, Benjamin Netanyahu, who said he was to talk to Trump on Wednesday evening, said in a video released by his office that the country was “prepared for all scenarios.”

Iran’s most senior negotiator, Mohammad Bagher Ghalibaf, remained defiant on Wednesday, saying Washington was seeking Tehran’s surrender through various means, including a naval blockade.

“The enemy, in its new design, is seeking, through a naval blockade, economic pressure and media manipulation, to destroy the country’s cohesion in order to force us to surrender,” Ghalibaf, the influential speaker of the Iranian parliament, said in a voice message published on his official Telegram channel.

Earlier on Wednesday Axios reported that Washington and Tehran were close to agreeing on a one-page memorandum of understanding to end the war.

The US-based news outlet reported that the US expected Iran to respond to several key points in the next 48 hours, and that while nothing had yet been agreed, this was the closest the parties had been to a deal.

Officials in Pakistan told the Guardian that an initial framework could possibly be agreed within 48 hours but that nothing was certain and that talks remained “difficult”.

Late on Tuesday, Trump abruptly ordered an indefinite pause to a naval effort to guide stranded commercial ships through the strait of Hormuz, which in normal times carries about a fifth of the world’s oil and liquid gas supplies.

More than 800 ships and roughly 20,000 crew members remain stranded west of the narrow waterway. Iran has threatened to deploy mines, drones, missiles and fast-attack craft, making passage through the strait too risky for commercial shipping and raising fuel prices around the world.

Trump wrote on social media that the decision to halt the new naval effort – called “Project Freedom” – just a day after it began came after requests from “mediator Pakistan and other countries”. He posted: “Great Progress has been made toward a Complete and Final Agreement.”

The US president said Washington’s blockade of Iranian ports, which is aimed at forcing Iran to make concessions in negotiations to end the war, would remain in place.

Trump, who is due to visit Beijing next week, has often threatened in recent weeks to restart the joint US-Israeli air offensive against Iran, but has also repeatedly indicated his desire for a negotiated end to the conflict.

Analysts suggested the US president may be seeking something that could be portrayed as a breakthrough before arriving in Beijing. China called on Monday for a comprehensive ceasefire in the Iran war.

Beijing has close economic and political ties to Tehran but has failed to exert significant leverage over the radical Islamist Iranian regime since the war began. The Trump administration may want China to exploit that relationship to convince Iran to open the strait.

Trump’s trip will be his first visit to China during his second term and the first by a US president since his previous visit, in 2017.

A senior Pakistani political source described “things … moving forward” with a focus in talks on obtaining a permanent ceasefire and “opening of the strait of Hormuz by both, at least for 60 days”.

The source added: “That gives a chance for both parties to talk out all important matters, including uranium enrichment. But nothing is finalised yet. Things are under discussion. We expect something to come out before the US president visits China.”.

Another Pakistani official said there was still a lack of trust between Iran and the US.

“There is still ambiguity in talks and nothing is completely decided. It is still 50/50 and things can go either way. The moment the US ends the blockade and the strait of Hormuz remains open, that will be the beginning of real talks. As long as there is a blockade from both sides, talks remain difficult. That’s why a framework of understanding to end the blockade for at least 30 to 60 days for further negotiations is important. It can be a trust-building measure,” the official said.

Analysts say that an external guarantor of any deal is essential.

“Pakistan and Iran both want China to become a guarantor but … Does China even have that power over both parties? Everyone has their doubts,” said the official.

The news of a possible deal sent the price of oil – which had jumped as much as 6% earlier in the week on the latest attacks in the Middle East – tumbling.

Many observers believe wide gaps remain between Iran and the US, making a durable ceasefire unlikely in the short term. The Iranian foreign ministry spokesperson, Esmaeil Baqaei said on Wednesday that Tehran would convey its position to key mediator Pakistan after “finalising its views”.

Later on Wednesday, Trump said in an interview with PBS he was optimistic about reaching an agreement with Iran before his trip to China.

“I think it’s got a very good chance of ending, and if it doesn’t end, we have to go back to bombing the hell out of them,” Trump told the broadcaster.

Trump insisted that under any deal Tehran would “export” its highly enriched uranium, necessary for making a nuclear weapon, to the US, a demand that, experts say, Iran cannot accept.

The Iran war, which began with an Israeli strike that killed Ayatollah Ali Khamenei, Iran’s then supreme leader, appears to have reached a stalemate. Iran is suffering huge economic losses, which may escalate if it begins to run out of storage capacity for its oil, but Trump is under pressure domestically and internationally as fuel prices surge in the US and globally.

US officials have insisted the ceasefire in the Middle East is holding though Monday saw an increase in violence, with Iranian missiles and drones striking the UAE for the first time for weeks, and reported clashes in the strait of Hormuz.

The US defence secretary, Pete Hegseth, said that the US had successfully secured a path through the waterway and that hundreds of commercial ships were lining up to pass through. “We know the Iranians are embarrassed by this ​fact. They said they control the strait. They do not,” Hegseth said.

Control of the strait and the threat to restart attacks on nearby countries’ oil and other infrastructure in the Gulf are the two main cards Iran can play in negotiations.

Both Washington and Tehran appear to believe they are close to victory and so have been unwilling to make significant concessions to allow the on-off talks to progress.

Rising petrol prices and a slowing global economy also pose a political threat to Trump as the US approaches congressional elections in November. A win for the Democrats in one or both chambers would weaken his presidency. Trump has so far shrugged off domestic concerns, with some reports suggesting he is more interested in securing what he sees as his legacy.