Pope Leo decried the use of military force as a means of achieving diplomatic goals on Friday, delivering an unusually fiery annual foreign policy speech in which he also called for human rights to be protected in Venezuela.

Leo, the first US pope, said the weakness of international organizations in the face of global conflicts was "a particular cause for concern."

"A diplomacy that promotes dialog and seeks consensus among all parties is being replaced by a diplomacy based on force," Leo told some 184 ambassadors accredited to the Vatican.

"War is back in vogue and a zeal for war is spreading," said Leo, who was elected pope in May.

Referring to the capture by US forces of Venezuelan President Nicolas Maduro on the orders of President Donald Trump last weekend, the pope called for world governments to "respect the will" of the Venezuelan people going forward.

Nations must "safeguard the human and civil rights" of Venezuelans, Leo added.

Leo's comments were part of an address that is sometimes called the pope's "state of the world" speech. It was the first given by Leo, who was elected following the death of Pope Francis.

Both the US and Venezuelan ambassadors to the Holy See were among those present for the event.

Leo, formerly the US Cardinal Robert Prevost, served as a missionary in Peru for decades before becoming pope. He has previously criticized some of Trump's policies, in particular on immigration, but did not mention the US president by name in Friday's speech.

Leo had shown a more muted, diplomatic tone in the first eight months of his papacy compared to his predecessor Francis, who often grabbed headlines with off-the-cuff comments.

But in Friday's 43-minute address, Leo used a more fiery tone - firmly condemning the world's ongoing conflicts, but also blasting the practices of abortion, euthanasia and surrogate births.

In unusually firm language for a pontiff, Leo also warned that freedom of expression is "rapidly shrinking" in Western countries.

"A new Orwellian-style language is developing which, in an attempt to be increasingly inclusive, ends up excluding those who do not conform to the ideologies that are fueling it," he said.

The pope also criticized what he called "a subtle form of religious discrimination" suffered by Christians in Europe and across the Americas.