Pope Leo XIV has said the prohibition on countries violating one another’s borders has been “completely undermined” in an important speech to diplomats gathered in the Vatican.

The first United States-born pope, giving his debut “state of the world” address, expressed his concern about “escalating tensions” in the “Caribbean Sea and the American Pacific coast” while calling for the “will of the Venezuelan people” to be respected and a return to stability in the country.

“War is back in vogue, and a zeal for war is spreading,” Leo told ambassadors from across the globe gathered in the Vatican’s Apostolic Palace on Friday. “The principle established after the Second World War, which prohibited nations from using force to violate the borders of others, has been completely undermined.”

Leo was speaking just less than a week after the US captured Venezuela’s leader, Nicholas Maduro, in a military raid, and as Russia continues to show little desire to end its war with Ukraine.

In his address, the American pope emphasised the importance of international law. His words made a striking contrast with those of US President Donald Trump, who told the New York Times that he felt constrained only by his “own morality”, while dismissing international law and the post-World War II order.

Leo lamented the weakness of “multilateralism” – the idea of countries working together to solve problems – and insisted on the “importance of international humanitarian law”, which he said must “always prevail over the ambitions of belligerents”.

 He pointed out that it was out of the tragedy of World War II that the United Nations was formed, for the safeguarding of peace, and warned that a diplomacy that “seeks consensus” is now being replaced by one “based on force, by either individuals or groups of allies.”

The Chicago-born pope spoke almost entirely in his native English for what was his first annual New Year’s address to the international diplomatic corps, who represent their countries to the Vatican.

Leo, wearing the red papal mozetta, described the encounter as “a new experience for me,” as he addressed a gathering which included Brian Burch, the US Ambassador to the Holy See, and Franklin Mauricio Zeltzer Malpica, the Venezuelan ambassador.

During a wide-ranging, 43-minute speech, Leo issued strong condemnations of abortion, surrogacy and euthanasia, defending the right of conscientious objection by doctors and healthcare professionals, from those refusing to participate in terminations and assisted deaths to those declining military service.