"Today... there was this threat against all the people of Iran, and this is truly unacceptable," the pope told journalists, without mentioning the United States, which is at war with the Islamic republic.

"There are certainly questions of international law, but much more than that, it is a moral question," he said as he left his residence in Castel Gandolfo, outside Rome, for the Vatican.

His comments came just hours ahead of Trump's 0000 GMT deadline.

The US president warned on Tuesday that "a whole civilization will die" if Iran does not open the Strait of Hormuz by then.

Iran has effectively closed the Strait -- a key transit route for Middle Eastern oil -- since the United States and Israel started the war in February, igniting a global energy shock.

Leo, the Catholic Church's first US pope, said that attacks on civilian infrastructure are "against international law".

He urged those involved to "come back to the table" for negotiations.

"Let's talk. Let's look for solutions in a peaceful way," he said.

The pontiff also urged "the citizens of all the countries involved to contact the authorities, political leaders, congressmen, to ask them, tell them to work for peace and to reject war always."

Since his election in May, the Chicago-born pontiff has taken a clear stand against some decisions by Trump's administration.

But faced with a highly polarised US political landscape, he has also relied on the American Catholic hierarchy to deliver the strongest criticism in his stead.

On Tuesday, the head of the US Conference of Catholic Bishops, Archbishop Paul Coakley, issued a direct appeal to Trump "to step back from the precipice of war".

"The threat of destroying a whole civilization and the intentional targeting of civilian infrastructure cannot be morally justified," he said in an open letter.

"There are other ways to resolve conflict between peoples.

"I call on President Trump to step back from the precipice of war and negotiate a just settlement for the sake of peace and before more lives are lost," Coakley said.