Washington DC - President Donald Trump said Thursday that Pope Leo XIV can say what he likes about world issues, but needs to understand the realities of a "nasty world."

The president and the US-born pontiff have squared off in recent days, with Trump roundly criticizing the leader of the world's 1.4 billion Catholics on everything from Iran to immigration.

Pope Leo, meanwhile, has said he has a "moral duty" to speak out against war and called Trump's threat to attack Iranian civilization "unacceptable."

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Trump was more conciliatory Thursday as he spoke to reporters before heading out west – but still sought to school the pope on the war in Iran.

"The pope has to understand Iran has killed more than 42,000 people over the last few months," the president said.

"They were totally unarmed protesters. The pope has to understand that. This is the real world, it's a nasty world."

Trump – who had dismissed the pope as "weak" and "wrong" in recent days – denied that he was "fighting" with the pontiff and said he had "nothing against" him.