Donald Trump unilaterally announced an extension of the two-week ceasefire with Iran on Tuesday amid frantic efforts to bring the two sides back to the negotiating table.

Hours after announcing that he “expected to be bombing”, the US president adopted a starkly different tone in a post on his Truth Social platform, saying he would extend the ceasefire until Iranian negotiators submitted a proposal for peace.

“Based on the fact that the Government of Iran is seriously fractured, not unexpectedly so and, upon the request of Field Marshal Asim Munir, and Prime Minister Shehbaz Sharif, of Pakistan, we have been asked to hold our Attack on the Country of Iran until such time as their leaders and representatives can come up with a unified proposal,” he wrote.

“I have therefore directed our Military to continue the Blockade and, in all other respects, remain ready and able, and will therefore extend the Ceasefire until such time as their proposal is submitted, and discussions are concluded, one way or the other.”

The declaration came in a topsy-turvy day in which an expected trip to Islamabad by JD Vance, the vice-president, had been put on hold and Trump ramped up his bellicose rhetoric, saying the US military was “raring to go”.

Trump’s sharp about-turn drew a withering early response from Mohammad Bagher Ghalibaf, the speaker of the Iranian parliament who has emerged as the Islamic regime’s lead negotiator in recent negotiations.

Quotes attributed to Ghalibaf’s personal adviser dismissed the ceasefire extension as “a ploy to buy time for a surprise strike”, adding that “the time for Iran to take the initiative has come”.

“The losing side cannot dictate terms,” the adviser, Mahdi Mohammadi, wrote on social media. “The continuation of the siege must be met with a military response.”

The negative response appeared to reflect suspicions among hardliners in Iran’s Islamic Revolutionary Guard Corps (IRGC) that Trump has been lying in his recent social media statements about fledgling agreements reached in negotiations.

Senior IRGC figures – who have the upper hand in Iran’s leadership – were angered by Trump’s flurry of social media posts last Friday, in which he all but proclaimed victory while depicting Iran as surrendering on key points, including its nuclear programme. Iranian anger led to the strait of Hormuz being re-closed a day after the foreign minister, Abbas Araghchi, had declared it open.

However, the Pakistani prime minister, Shehbaz Sharif – who has acted as the principal mediator – thanked Trump. “Pakistan shall continue its earnest efforts for [a] negotiated settlement of [the] conflict,” he posted.

The US president had earlier told the US business news network CNBC that he did not want to extend the ceasefire with Tehran, insisting the US was in a strong position and was “going to end up with a great deal”. Trump has previously said that targets for new US attacks would include power stations and other civilian infrastructure.

Iran appeared unwilling to bend to Trump’s threats, though analysts say there is fierce disagreement among its leaders over how to respond to US pressure and whether to risk a potentially devastating new wave of bombing.

Iranian state television on Tuesday broadcast a message confirming that “no delegation from Iran has visited Islamabad … so far” and Iran’s chief negotiator, the parliamentary speaker Mohammad Bagher Ghalibaf, accused the US president of seeking to turn the negotiating table into a “table of surrender”.

“We do not accept negotiations under the shadow of threats,” he wrote in a social media post, and said Iran was preparing “to reveal new cards on the battlefield”.

There were also differences over when the ceasefire expires, with Iran saying it was in the early hours of Wednesday in Iran, and Trump putting it at the end of Wednesday in Washington. Tehran is 7.5 hours ahead of Washington.

A first round of talks in Islamabad 10 days ago ended with no sign of agreement on the future of the strait of Hormuz, the key waterway which was closed to shipping by Iran in the early days of the conflict, cutting the supply of around a fifth of the world’s oil and gas.

Fatih Birol, the head of the International Energy Agency, said the combined impact of the conflict’s effects on oil, alongside the effects of Russia’s war with Ukraine on gas supplies, was “the biggest crisis in history” in global energy markets.

The US last week imposed a blockade on Iranian ports to pressure Tehran into reopening the strait, and on Sunday it seized an Iranian cargo vessel.

US forces then escalated the campaign on Tuesday, boarding an oil tanker previously under sanctions for smuggling Iranian crude oil in Asia. Ship-tracking data showed the vessel in the Indian Ocean between Sri Lanka and Indonesia around the time it was intercepted.

Iran’s foreign ministry spokesperson Esmaeil Baghaei said on Iranian state TV that US moves against the two vessels amounted to “piracy at sea and state terrorism” and questioned Washington’s seriousness in negotiating.

The closure of the strait by Iran threatens a global recession and has given Tehran a powerful strategic weapon to counter the overwhelming conventional military superiority of its enemies. The war began in February with a first wave of bombing by the US and Israel, which killed the then supreme leader of Iran, Ayatollah Ali Khamenei.

Yvette Cooper, the UK’s foreign secretary, who has been holding discussions with counterparts aimed at safeguarding the strait, has described it as “a critical diplomatic moment” in the crisis.

In Islamabad, Pakistani officials have expressed confidence that Iran will resume talks in what are the highest-level negotiations between the two countries since the 1979 Islamic Revolution.

A spokesperson said the Pakistani foreign minister, Ishaq Dar, met on Tuesday with the acting US ambassador in Islamabad to urge a ceasefire extension. Dar also met the ambassador from China, which is a key trading partner with Iran.

“Pakistan has made sincere efforts to convince the Iranian leadership to participate in the second round of talks, and these efforts continue,” Pakistan’s information minister, Attaullah Tarar, said on X.

Security has been tightened across Pakistan’s capital, where authorities have deployed thousands of personnel and increased patrols along routes leading to the airport. Government offices, schools and colleges in the city have been shut down and much of the centre barred to civilians.

“If they don’t come to Islamabad, or the second round does not take place, it will be an embarrassing situation for Pakistan as well,” Nusrat Javed, a political analyst and columnist, said.

Over the weekend, Iran said it had received new proposals from Washington, but also suggested a wide gap remains between the sides. Issues that derailed the last round of negotiations included Iran’s nuclear enrichment programme, its support for a series of militant movements that act as regional proxies, and the strait of Hormuz.

Trump said Iran had no choice and would take part in talks. “We’ve taken out their navy, we’ve taken out their air force, we’ve taken out their leaders.”

The US president again claimed “regime change” and said those now in charge were “much more rational”.

Many experts say the conflict has led to a radicalisation of Iran’s regime, with more pragmatic figures having been killed or sidelined, allowing senior officials in the Islamic Revolutionary Guard Corps to increase their hold on power.