In a New Year's message to diplomats at the Vatican, his first since becoming head of the world's 1.4 billion Catholics last May, Leo lamented the weakness of multilateralism, saying dialogue was being replaced by a "diplomacy based on force".

He did not name the US but said rising tensions in the Caribbean and Pacific were "cause for serious concern", particularly in Venezuela -- where US forces recently seized the president.

He was speaking just hours after US President Donald Trump said land operations against drug cartels were on the way following maritime attacks in the eastern Pacific Ocean and Caribbean Sea.

Leo issued an appeal "to respect the will of the Venezuelan people, and to safeguard the human and civil rights of all", following Trump's suggestion the US could effectively control Venezuela -- and its vast oil reserves -- for years.

The pope's address rounded up a year of increasing global tension, from the Russia-Ukraine war to US strikes in several countries, and rising fears in Europe of a possible US takeover of Greenland.

The pope warned that the principle established after the Second World War by which countries cannot use force to violate the borders of others "has been completely undermined".

Diplomacy based on dialogue "is being replaced by a diplomacy based on force, by either individuals or groups of allies".

"War is back in vogue and a zeal for war is spreading," he said.

- 'Grave threat' -

The pope said peace was being "sought through weapons as a condition for asserting one's own dominion".

"This gravely threatens the rule of law, which is the foundation of all peaceful civil coexistence," he said.

The Chicago-born pontiff said adhering to international law was "a commitment that states have made", and it "must always prevail over the ambitions of belligerents".

The destruction of hospitals, energy infrastructure, homes and places essential to daily life was "a serious violation" of international humanitarian law, he said.

Leo urged the international community to remember that protecting human dignity and the sanctity of life "always counts for more than any mere national interest", he said.

The pontiff pointed to the "weakness of multilateralism" as "a particular cause for concern".

He said its purpose was to provide a place where people could meet and talk, modelled on the ancient Roman Forum or medieval town square.

But it is difficult to talk "when words lose their connection to reality, and reality itself becomes debatable and ultimately incommunicable".

"Rediscovering the meaning of words is perhaps one of the primary challenges of our time," he said.

Leo also warned of the "danger of returning to the race of producing ever more sophisticated new weapons", particularly those that use artificial intelligence.

Recalling ongoing conflicts around the world, the pope repeated his support for a two-state solution in the Middle East, and lamented rising violence in the occupied West Bank.

"Sadly, there has been an increase in violence in the West Bank against the Palestinian civilian population, which has the right to live in peace in its own land," he said.