Pope Leo on Wednesday praised the two-week ceasefire in the Iran war, hours after calling U.S. President Donald Trump's threat against the population of Iran "unacceptable".

The pope, who has emerged as an outspoken critic of the war in recent weeks, said he welcomed the announcement of the ceasefire with satisfaction and urged continued negotiation to bring a full end to the regional conflict.

"In light of these past hours of great tension for the Middle East and for the whole world, I welcome with satisfaction... the announcement of an immediate two-week truce," Leo, the first U.S. pope, said in his weekly audience.

"Only through a return to negotiation can an end to the war be achieved," he said.

Leo, who is known for choosing his words carefully, has been ramping up his criticism of the U.S.-Israeli war against Iran.

After Trump threatened Iran on Tuesday that "a whole civilization will die tonight" if the country did not meet U.S. demands for a truce, Leo told journalists that threat was "truly unacceptable".

It is rare for the pope, who leads 1.4 billion Catholics around the world, to respond directly to a world leader.

-Reporting by Joshua McElwee, editing by Giulia Segreti and Toby Chopra/Reuters