The US and Iran agreed to a two-week conditional ceasefire on Tuesday evening after a last-minute diplomatic intervention led by Pakistan, canceling an ultimatum from Donald Trump for Iran to surrender or face widespread destruction.

Trump’s announcement of the ceasefire agreement came less than two hours before the US president’s self-imposed 8pm Eastern time deadline to bomb Iran’s power plants and bridges in a move that legal scholars, as well as officials from numerous countries and the Pope, had warned could constitute war crimes.

Just hours earlier, Trump had written on Truth Social: “A whole civilization will die tonight, never to be brought back again. I don’t want that to happen, but it probably will.” B-52 bombers were reported to be en route to Iran before the ceasefire agreement was announced.

But by Tuesday evening, Trump announced that a ceasefire agreement had been mediated through Pakistan, whose prime minister Shehbaz Sharif had requested the two-week peace in order to “allow diplomacy to run its course”.

Trump wrote in a post that “subject to the Islamic Republic of Iran agreeing to the COMPLETE, IMMEDIATE, and SAFE OPENING of the Strait of Hormuz, I agree to suspend the bombing and attack of Iran for a period of two weeks”.

In the two weeks, Trump said, he believed the US and Iran could negotiate over a 10-point proposal made by Tehran that would allow an armistice to be “finalized and consummated”.

“This will be a double sided CEASEFIRE!” he continued. “The reason for doing so is that we have already met and exceeded all Military objectives, and are very far along with a definitive Agreement concerning Longterm PEACE with Iran, and PEACE in the Middle East.”

Iran’s foreign minister, Abbas Araghchi, issued a statement shortly after Trump’s announcement saying Iran had agreed to the ceasefire.

“For a period of two weeks, safe passage through the Strait of Hormuz will be possible via coordinating with Iran’s Armed Forces,” he wrote.

The sudden about-face will allow Trump to step back as the US war in Iran has dragged on for five weeks with little sign that Tehran is ready to surrender or release its hold on the strait of Hormuz, a conduit for a fifth of the global energy supply, where traffic has slowed to a trickle.

Israel will also agree to the two-week ceasefire, Axios reported, citing an Israeli official, adding that the ceasefire would enter effect as soon as the blockade of the strait of Hormuz ceased.

Trump had earlier rejected the 10-point plan as “not good enough” but the president has set deadlines before and allowed them to pass over the five weeks of the conflict. Yet he insisted on Tuesday the ensuing hours would be “one of the most important moments in the long and complex history of the World” unless “something revolutionarily wonderful” happened, with “less radicalized minds” in Iran’s leadership.

Amir-Saeid Iravani, Iran’s representative at the UN, said that Trump’s threats constituted “incitement to war crimes – and potentially genocide”.

During a security council session on the strait of Hormuz, Iravani said: “Iran will not stand idle in the face of such egregious war crimes. It will exercise, without hesitation, its inherent right of self-defence and will take immediate and proportionate reciprocal measures.”

Through his spokesperson, the UN secretary-general, António Guterres, issued a reminder on Monday that attacking civilian infrastructure is banned under international law, but Trump declared on the same day he was “not at all” concerned about being called a war criminal.

In the hours before Trump’s deadline, Israel mounted its own attacks on Iran’s infrastructure. A rail bridge in the central city of Kashan was one of the first reported bombed on Tuesday by Iranian state media, with two people reportedly killed as Israel’s military said it had launched “a wide-scale wave of strikes targeting dozens of infrastructure sites”.

A bridge over a railway line near Karaj, to the north-west of Tehran, was hit, according to Iranian media, and power outages were reported in the same city after a substation and transmission lines were bombed. Bridges near Qom and Tabriz were also reportedly hit.

The US also struck 50 military targets on Iran’s Kharg Island, the home to its main oil export terminal, while Iran’s Revolutionary Guards said they had attacked Saudi Arabia’s Jubail petrochemical complex in retaliation for strikes on an Iranian petrochemical facility the night before.