Boris Johnson has spoken out against Donald Trump’s threat to bomb 52 Iranian cultural sites if the crisis escalates, in a rare crack in the transatlantic alliance.

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

The warning comes as Mr Johnson prepares to convene a meeting of the National Security Council (NSC) on Tuesday with senior ministers, officials and figures from the intelligence agencies into the escalating tensions with Tehran.

However, Downing Street declined to say whether such an attack would be a war crime – and gave stronger backing to the assassination of Qassem Soleimani than in Mr Johnson’s comments on Sunday.

Asked whether the UK accepted Washington’s justification for the attack, the spokesperson said: “States have a right to take action such as this in self-defence and the US have been clear that Soleimani was plotting imminent attacks on American diplomats and military personnel.”

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

On Sunday, Mr Johnson had stopped short of echoing his foreign secretary’s backing for Washington’s “right” to carry out the airstrike at Baghdad Airport, on Friday.

Following the non-binding vote of the Iraqi parliament for the removal of US troops, Mr Johnson also pressed for coalition forces – including the UK – to be allowed to remain in the country to continue operations against the Islamic State terror group during a phone call with his Iraqi counterpart Adil Abdul Mahdi​.

“We urge the Iraqi government to ensure that the coalition is able to continue its vital work countering this shared threat,” said the spokesperson.

But asked if the UK would regard attacks on cultural sites as a war crime, the spokesperson replied only that details of the international agreement were contained in the 1954 Hague Convention for the Protection of Cultural Property in the Event of Armed Conflict.

The prime minister also met with Dominic Raab, the foreign secretary, defence secretary Ben Wallace and senior officials including the cabinet secretary at Downing Street this afternoon to discuss the crisis in Iran.

Following the meeting, Mr Raab said the government has changed its travel advice for UK citizens travelling to the region and that “we are going to be reinforcing in due course the royal navy protection for shipping in the Strait of Hormuz”.

He added: “Clearly our first priority is to make sure that UK nationals, citizens, shipping, diplomatic missions and military personnel are safe.

“On a diplomatic front our overwhelming message that the prime minister and I are conveying to our European and American counterparts, and also critically our partners in the Middle East, is the importance of de-escalating the tensions and finding a diplomatic way through this crisis.”

The PM’s spokesperson also brushed off suggestions that Mr Johnson was slow to respond to the situation during his luxury holiday in Mustique after Emily Thornberry, the shadow foreign secretary, accused him of “sunning himself and drinking vodka martinis” in the Caribbean instead of dealing with the crisis.

“The prime minister was in contact with senior officials and senior ministers throughout the course of Friday, Saturday and Sunday,” the spokesperson insisted.

“The government’s position was set out by the foreign secretary on Friday morning and there were regular updates from both the foreign secretary and the defence secretary.”

Responding to criticism that the UK was not informed in advance of the plans to target General Soleimani, the spokesperson said: “We have a very close security partnership with the US. We have regular dialogue at every level.”

Iran’s announcement that it would no longer respect the terms of the international deal restricting its nuclear activities was described as “clearly extremely concerning”.

The UK was currently in discussions with fellow signatories about “next steps”, the spokesperson said, in comments which will fuel expectation of sanctions on Tehran if it presses ahead with nuclear enrichment for military purposes.

He added: “It is in everyone’s interests that the deal remains in place. It makes everyone safer by taking the prospect of a nuclear-armed Iran firmly off the table.