The US has launched new airstrikes against Iran, hours after Donald Trump threatened to escalate the conflict unless Iran stopped attacking ships in the strait of Hormuz.

Iran, which is burying its former supreme leader Ayatollah Ali Khamenei on Thursday, responded by targeting US-allied Kuwait and Qatar and accused the US of striking near its sole nuclear power plant.

The tit-for-tat strikes were the largest since Tehran and Washington signed a memorandum of understanding (MoU) on 17 June aimed at extending the ceasefire and giving space for negotiations for a permanent truce.

Sirens sounded at least three times in Bahrain, where the US navy’s fifth fleet is headquartered. Iran’s Islamic Revolutionary Guard Corps said it fired 10 ballistic missiles at Jordan’s Azraq military base on Thursday afternoon, which the US air force uses. There were no immediate reports of any damage in Jordan or the Gulf.

In Iran, officials said strikes targeted the perimeter of Iran’s only civilian nuclear plant in Bushehr province, an area where the UN’s nuclear watchdog has previously warned that attacks could “pose a very real danger to nuclear safety”.

“Several areas in Bushehr province were targeted today, including the perimeter of the nuclear power plant, a military base in the town of Choghadak and a fishing pier in the south of the province,” said Ehsan Jahanian, the deputy governor of Bushehr, adding there were no reports of casualties so far.

The attacks came hours after Trump said the MoU was “over” because of Iranian attacks on ships in the strait of Hormuz. After the strikes, the US president posted videos of explosions in Iran and threatened the country once again.

“This is in retribution for yesterday’s bombing of ships by Iran. If it happens again, it will get much worse!” Trump wrote on Truth Social. Hours before, he had promised strikes would not lead to long-term fighting but would be “very fast”.

His comments and the exchange of fire prompted worries that the ceasefire could break down and raised concerns about the long-term prospects of negotiations. Significant gaps remain between the two countries over issues such as Iran’s control over the strait, as well as inspections of nuclear facilities.

Back-and-forth attacks have happened since the MoU was signed, but Thursday’s strikes were the most intense in weeks. The attacks caused oil prices to rise, before they recovered on Thursday after calm was restored.

They came as Iran prepared to bury Khamenei in his home town of Mashhad. A fighter jet escorted the plane carrying Khamenei’s coffin amid the tense security situation. Crowds of people gathered for the occasion, some holding banners promising vengeance against the US.

Over 10 million people participated in a funeral procession for the late leader in Iraq, according to an Iraqi official, while thousands rallied in the southern suburbs of Beirut in a ceremony organised by Hezbollah on Wednesday night.

Observers waited to see if Iran’s new supreme leader, Mojtaba Khamenei, would attend the burial. He was wounded in the same series of strikes that killed his father and has since only communicated through written statements.

The US military said it hit about 90 targets in Iran, showing footage of strikes on missile launchers and a runway. It said the attacks were meant to degrade Iran’s capacity to “threaten freedom of navigation” in the strait of Hormuz, a critical choke point for about 20% of the world’s oil and gas.

Iran accused the US of war crimes after it said two bridges in the eastern provinces leading to the city of Mashhad were targeted. The bridges constitute key infrastructure for Iran’s cross-border trade with China, which has dramatically increased since the start of the war.

Trump has repeatedly threatened to hit bridges, power plants and other civilian infrastructure in Iran. Targeting civilian infrastructure if it is not a military objective could amount to a war crime.

Iranian state media also reported explosions in several cities, including Bushehr, which houses Iran’s nuclear power plant complex. At least three people were killed in Iran’s south-western Khuzestan province, while a firefighter was killed in an airport in the south-eastern city of Iranshahr. Nine members of Iran’s military also died in strikes on Wednesday.

The US attack came after Iran struck three ships in the strait of Hormuz on Tuesday. Iran stopped virtually all shipping out of the strait during the war, sending global energy prices soaring and putting pressure on Trump domestically before midterm elections.

The MoU calls for the reopening of the strait to commercial shipping for 60 days. Iran says it wants to charge fees to ships transiting through the strait, conflicting with the US, which says it is an international waterway and should not have tolls.

Iran continues to view its control over the strait as an important source of leverage in its negotiations with the US, while Trump appears to view strikes on Iran as a way of increasing pressure on Tehran.

Iran’s navy claimed in a statement that the number of ships transiting the strait had been restored to 50% of prewar levels and that permission to go through the strait was only being given to ships using routes designated by Iran. It further warned that any further US military action would trigger a “crushing response.”

“The adventurous actions of the American terrorist army and their interference in determining traffic routes, in addition to provoking a decisive response, will also seriously disrupt the gradual reopening process and jeopardize the interests of countries that benefit from the strait of Hormuz,” the navy said in a statement.

Mohammad Bagher Ghalibaf, a senior Iranian negotiator and parliament speaker, said US pressure would not lead anywhere. “America still hasn’t learned that bullying and breaking promises are no longer cost-free,” he said in a post on X. “Let me put it plainly: if you strike, you’ll get hit.”

Mediators attempted to de-escalate tensions between the US and Iran in an effort to salvage negotiations. The Qatari prime minister, Sheikh Mohammed bin Abdulrahman Al Thani, a key intermediary between the countries, spoke to Iran’s foreign minister, Abbas Araghchi, on Thursday, condemning Tehran’s strikes on ships in the strait.

Negotiations towards reaching a final deal were intended to start after the conclusion on Thursday of Khamenei’s seven-day funeral.

Additional reporting by Patrick Wintour