UK Prime Minister Boris Johnson has warned US President Donald Trump against targeting Iranian cultural sites.

A representative for Johnson says doing so, as Trump has repeatedly threatened, would breach international law.

Johnson and other European leaders released a joint statement on Sunday calling on the US and Iran to step back from outright war.

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UK Prime Minister Boris Johnson has warned US President Donald Trump that any attempt to target Iranian cultural sites would be a breach of international law.

Trump threatened multiple times over the weekend to target Iranian cultural sites should Iran retaliate for the killing of Maj. Gen. Qassem Soleimani, tweeting that the US could hit such sites “VERY FAST AND VERY HARD.”

“They’re allowed to kill our people,” Trump told reporters on Sunday night. “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.”

Responding to the comments, a representative for Johnson said on Monday that any attempt to target Iranian cultural sites would breach international law.

“There are international conventions in place that prevent the destruction of cultural heritage,” the person said.

Asked whether Johnson regarded such attacks as a war crime, the representative said destroying cultural sites was forbidden under the terms of the Hague Convention, which governs the conduct of wars and of which the US is a signatory.

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“Well as I say, you can read the international conventions for themselves,” the person said. “It is the 1954 Hague Convention for the protection of cultural property in the event of armed conflict.”

European leaders on Sunday evening issued a statement calling for the US and Iran to show “the utmost restraint and responsibility.”

“There is now an urgent need for de-escalation,” Johnson, German Chancellor Angela Merkel, and French President Emmanuel Macron said in a joint statement.

“We call on all parties to exercise utmost restraint and responsibility. The current cycle of violence in Iraq must be stopped.”

The UK also previously called on Trump to step back from all-out war with Iran.

The foreign secretary, Dominic Raab, said on Friday that conflict with Iran “is in none of our interests” and urged “all parties to de-escalate.”

The crisis has strained relations between the two allies. US Secretary of State Mike Pompeo on Friday accused the UK and other European allies of “not being helpful” over the crisis.