Donald Trump’s threat to destroy Iranian cultural sites would be a war crime if carried out, legal experts have claimed, as the US president doubled down on his comments over the weekend.

In the aftermath of the US killing of top Iranian general Qassem Soleimani in Iraq last week, Mr Trump warned Iran that a list of targets had been drawn up should it retaliate, including cultural sites with no military value.

“Let this serve as a WARNING that if Iran strikes any Americans, or American assets, we have.... targeted 52 Iranian sites (representing the 52 American hostages taken by Iran many years ago), some at a very high level & important to Iran & the Iranian culture, and those targets, and Iran itself, WILL BE HIT VERY FAST AND VERY HARD,” he wrote on Twitter.

International law prohibits deliberate attacks on civilian objects not being used for military purposes. Experts in international law said the attacks, if carried out, would constitute war crimes.

“President Trump should publicly reverse his threats against Iran’s cultural property and make clear that he will not authorise nor order war crimes,” said Andrea Prasow, acting Washington director at Human Rights Watch.

Ms Prasow added that the threat demonstrated the president’s “callous disregard for the global rule of law”.

“The US Defence Department should publicly reaffirm its commitment to abide by the laws of war and comply only with lawful military orders,” she said.

US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Show all 35 1 /35 US airstrike kills Iran's Qassem Soleimani: Fallout in pictures US airstrike kills Iran's Qassem Soleimani: Fallout in pictures This photo released by the Iraqi Prime Minister Press Office shows a burning vehicle at the Baghdad International Airport following an airstrike in Baghdad, Iraq, early Friday 3 January AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures The wreckage of the car in which general Soleimani was travelling when a targeted US airstrike struck outside Baghdad International Airport on 3 January Ahmad Al Mukhtar via Reuters US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Demonstrators burn the US and British flags during a protest in Tehran after general Soleimani was killed in a targeted airstrike by American forces Reuters US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A burning vehicle at the Baghdad International Airport following an airstrike. The Pentagon said Thursday that the US military has killed general Qassem Soleimani, the head of Iran's elite Quds Force, at the direction of Donald Trump AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Protesters burn Israeli and US flags as thousands of Iranians take to the streets to mourn the death of general Soleimani at the hands of America EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Supporters of Donald Trump pray at an 'Evangelicals for Trump' campaign event held on the day following the killing of general Soleimani. At the event, the president praised the "flawless strike that eliminated the terrorist ringleader" AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A huge procession of mourners gather in Baghdad for the funeral of general Soleimani on 4 January AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Thousands of Iranians take to the streets to mourn the death of Soleimani during an anti-US demonstration to condemn the killing of Soleimani, after Friday prayers in Tehran, Iran EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iraqis perform a mourning prayer for slain major general Qasem Soleimani of the Iranian Revolutionary Guards at the Great Mosque of Kufa AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A billboard reading 'Death to America and Israel', installed by Iran-backed shiite armed groups at a street in Jadriyah district in Baghdad, Iraq EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A handout picture provided by the office of Iran's supreme leader Ayatollah Ali Khamenei shows him visiting the family of Soleiman KHAMENEI.IR/AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Thousands of Iranians take to the streets in Tehran EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Pakistani Shiite Muslims burn a mock of a US flag as they hold pictures of General Qasem Soleimani during a protest against the USA, outside the US Consulate in Lahore, Pakistan EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iran's Ambassador to Lebanon Mohammed Jalal Feiruznia, looks to a portrait of Soleimani, as he receives condolences at the Iranian embassy, in Beirut, Lebanon AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures People make their way on the street while a screen on the wall of a cinema shows a portrait Soleimani in Tehran AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Aziz Asmar, one of two Syrian painters who completed a mural following the killing of Iranian Revolutionary Guards commander Qasem Soleimani poses next to his creation in the rebel-held Syrian town of Dana in the northwestern province of Idlib AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A demonstration in Tehran AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures An anti-US demonstration to condemn the killing of Soleimani, after Friday prayers in Tehran EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Mujtaba al-Husseini, the representative of Iran's supreme leader Ayatollah Ali Khamenei, delivers a speech in the holy shrine city of Najaf AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Pakistani Shiite Muslims burn a mock of a US and Israeli flags as they hold pictures of General Qasem Soleimani during a protest against the USA, outside the US Consulate in Lahore, Pakistan EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Protesters demonstrate in Tehran AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Pakistani Shi'ite Muslims hold pictures of General Qasem Soleimani during a protest against the USA, in Peshawar, Pakistan EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Protesters, holding a photograph of the leader of the People's Mujahedin of Iran Massoud Rajavi, outside Downing Street in London PA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Protesters burn a US flag in Tehran AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A Syrian man offers sweets to children to mark the killing AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iranian worshippers attend a mourning prayer for Soleimani in Iran's capital Tehran AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Kashmiri Shiite Muslims shout anti American and anti Israel slogans during a protest AP US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iranian worshipers chant slogans during Friday prayers Reuters US airstrike kills Iran's Qassem Soleimani: Fallout in pictures A protest against the USA, in Islamabad, Pakistan EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iranians burn a US flag in Tehran EPA US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Supporters of the National Council of Resistance of Iran in Germany (NWRI) protest outside Iran's embassy in Berlin, Germany Reuters US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Supporters of the National Council of Resistance of Iran in Germany (NWRI) protest outside Iran's embassy in Berlin Reuters US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iranian worshippers in Tehran AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Vehicles of the United Nations Interim Forces in Lebanon (UNIFIL) patrol a road in the southern Lebanese town of Kfar Kila near the border with Israel. Following morning's killing of Major General Qasem Soleimani, Lebanon's Iran-backed Hezbollah movement called for the missile strike by Israel's closest ally, to be avenged AFP via Getty US airstrike kills Iran's Qassem Soleimani: Fallout in pictures Iranian women take to the streets in Tehran EPA

Iran has vowed to retaliate for the death of Soleimani, the leader of Iran’s Quds Force, who was killed in a US drone strike at Baghdad airport last Friday. Mr Trump ordered the strike against Soleimani following weeks of tensions between the US military and Iran-backed militias in Iraq.

The killing, and the president’s subsequent threats, have provoked a flood of criticism at home and abroad.

Boris Johnson issued a rare criticism of the UK’s transatlantic ally on Monday.

“There are international conventions in place that prevent the destruction of cultural heritage,” the prime minister’s spokesperson said.

“You are threatening to commit war crimes,” said Senator Elizabeth Warren, a frontrunner in the Democratic party’s list of candidates hoping to challenge Mr Trump in November’s election.

“We are not at war with Iran. The American people do not want a war with Iran,” she added.

Brett McGurk, former US envoy to the international coalition set up to defeat Isis, called Mr Trump’s comments “unacceptable” and “unAmerican”.

“American military forces adhere to international law. They don’t attack cultural sites. And they’re not mercenaries. Reckless and unprecedented words from a commander-in-chief,” he said.

Iran’s foreign minister, Mohammad Javad Zarif, tweeted that “targeting cultural sites is a WAR CRIME”.

“Having committed grave breaches of int’l law in Friday’s cowardly assassinations, @realdonaldtrump threatens to commit again new breaches of JUS COGENS [international law],” he added.

Despite the criticism, Mr Trump doubled down on his threat on Sunday.

“They’re allowed to kill our people. They’re allowed to torture and maim our people. They’re allowed to use roadside bombs and blow up our people. And we’re not allowed to touch their cultural sites? It doesn’t work that way,” the president said in comments to reporters.

The president also raised the prospect of using disproportionate force to respond to any Iranian retaliation – which would also constitute a war crime.

“These Media Posts will serve as notification to the United States Congress that should Iran strike any U.S. person or target, the United States will quickly & fully strike back, & perhaps in a disproportionate manner. Such legal notice is not required, but is given nevertheless!” he wrote.

US secretary of state Mike Pompeo appeared to offer a confusing defence of the president’s words in an interview with CNN.

“If we need to defend American interests, we will do so. What President Trump said last night is consistent with what we have said all along,” he told CNN’s Jake Tapper.

“And the American people should know we will always defend them and we’ll do so in a way that is consistent with international rule of law and the American constitution,” he added, without explaining how targeting cultural sites would not violate international law.

Iran's Supreme Leader Ayatollah Ali Khamenei weeps at Soleimani prayers

The Geneva Conventions specifically prohibit “any acts of hostility against cultural objects, including making such objects the target of reprisals”, according to Human Rights Watch.

The rights group also noted that the US is a party to the Hague Convention for the Protection of Cultural Property in the Event of Armed Conflict (1954), which similarly prohibits such attacks.

The United Nations' cultural agency, Unesco, addressed the controversy with a reminder that the US had signed treaties committing to not harming cultural heritage in the event of armed conflict.

Unesco director general Audrey Azoulay said the US had signed a 1972 convention that obliged it not to undertake "any deliberate measures which might damage directly or indirectly the cultural and natural heritage" of other states.

Rhys Davies, an international criminal law barrister, told The Independent that the kind of military action described in Mr Trump’s tweets “could certainly be a war crime”, but that any kind of prosecution might be difficult.

“Even though the US is not a signatory of the International Criminal Court, there is a domestic framework where someone could be brought to justice in the US. But that probably enters the realm of the political.

As the leader of Iran’s Quds Force, 62-year-old Soleimani was the architect of Iran’s expanding influence across the Middle East. He travelled the region in his dual roles of diplomat and military commander to build and support a “resistance bloc” – an alliance of Shia militant groups that would facilitate Iran’s regional ambitions and counter US influence.