Pope Francis, in some his strongest comments ever on nuclear weapons, has said the world should condemn not only their possible use but "their very possession".

The appeal came at the start of a two-day conference on nuclear disarmament that has brought together 11 Nobel Peace Prize winners, as well as United Nations and NATO officials, to discuss prospects for a world free of nuclear weapons.

Addressing the group, he spoke of "the catastrophic humanitarian and environmental effects of any employment of nuclear devices".

He added: "If we also take into account the risk of an accidental detonation as a result of error of any kind, the threat of their use, as well as their very possession, is to be firmly condemned."

The Vatican has urged world leaders to shun any unilateral action and seek dialogue to respond to the threat of nuclear weapons.

"Growing interdependence means that any response to the threat of nuclear weapons should be collective and consultative, based on mutual trust," Vatican Secretary of State Cardinal Pietro Parolin said.

As tensions between the United States and North Korea have increased in recent months, Pope Francis and the Vatican have warned that a nuclear conflict would destroy a good part of humanity.

"This trust can be built only through dialogue that is truly directed to the common good and not to the protection of veiled or particular interests," said Cardinal Parolin, who ranks second only to the pope in the Vatican hierarchy.

"Such dialogue, as far as possible, should include all. Avoiding conflicts and building bridges - this should be the principle aim of an efficacious, collective and consultative response," he said.

While Cardinal Parolin did not mention North Korea or US President Donald Trump, another participant at the conference, Cardinal Peter Turkson, said the world was in "complex and uncertain times".

"By the way, when we planned this conference, we did not know that President Trump would be in the Far East. It just happens to be a happy coincidence," he added.

"I guess it has to do with divine providence."