Boris Johnson warns Trump against targeting Iranian cultural sites.
A spokesperson for Johnson says doing so would be a breach of international law.
Trump on Monday repeated his threat to target the sites.
Johnson and other European leaders release a joint statement calling the US and Iran to step back from outright war.
Boris Johnson has warned Donald Trump that any attempt to target Iranian cultural sites would be a breach of international law.
Trump on Monday repeated his threat to target Iranian cultural sites, after suggesting over the weekend that the US could hit the sites "very fast and very hard."
"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," Trump told reporters.
Responding to the comments, a spokesperson for Johnson said on Monday that any attempt to target Iranian cultural sites would be a breach of international law.
"There are international conventions in place that prevent the destruction of cultural heritage," the spokesperson said.
Asked if Johnson regarded such attacks as a war crime, the spokesperson repeated that they would be a breach of international conventions.
However, he added that destroying cultural sites is forbidden under the terms of the Hague Convention, which governs the conduct of wars, and which the US is a signatory.
The comments came after European leaders issued a statement calling for both the US and Iran to show "the utmost restraint and responsibility."
"There is now an urgent need for de-escalation," Johnson, Angela Merkel and Emmanuel Macron said in a joint statement on Sunday evening.
"We call on all parties to exercise utmost restraint and responsibility. The current cycle of violence in Iraq must be stopped."
The statement came after the UK 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 countries. US Secretary of State Mike Pompeo on Friday. accused the UK and other European allies of "not being helpful," over the ongoing crisis.