VATICAN CITY (Reuters) - Pope Francis appeared to harden the Catholic Church’s teaching against nuclear weapons on Friday, saying countries should not stockpile them even for the purpose of deterrence.

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His remarks, at the start of a disarmament conference that brought 11 Nobel Peace Prize winners to the Vatican, appeared to go further than previous popes. They have said that while nuclear weapons should never be used, holding arsenals solely to deter other countries from using them could be morally acceptable as a step toward achieving a nuclear-free world.

Addressing the group in the 16th century frescoed Clementine Hall of the Vatican’s Apostolic Palace, Francis 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.”

As tensions between the United States and North Korea have increased, the pope has often warned that a nuclear conflict would destroy a good part of humanity and called for a third country to mediate the dispute.

In his address, Francis did not directly mention the North Korea tension, but spoke in general of a “climate of instability and conflict” and a “mentality of fear” in the world today.

“International relations cannot be held captive to military force, mutual intimidation, and the parading of stockpiles of arms,” he said.

He also said international laws against proliferation of nuclear weapons had not kept new states from acquiring them. Money used to develop or modernize weapons should instead be spent on helping the poor and protecting the environment.

Douglas Roche, Canada’s former Ambassador for Disarmament and a former senator, told the conference the pope’s remarks against possession of nuclear weapons were “historic” and asked national conferences of Catholic bishops to work to make it known.

Another participant suggested the pope should write an encyclical letter addressed to all Catholics on the moral imperative to ban nuclear weapons.

Among those who met the pope were Beatrice Fihn, executive director of the International Campaign to Abolish Nuclear Weapons (ICAN) and Mohamed El Baradei, director general emeritus of the International Atomic Energy Agency (IAEA).

Fihn, whose group won the Nobel this year, told Reuters she asked the pope to lead all 1.2 billion Catholics around the world in prayer for an end to the threat of nuclear weapons on Sunday, December 10, when her group is due to collect the prize.

“Tensions are really high and the risks for nuclear weapons’ use is higher than at the height of the Cold War, the Cuban missile crisis. I think that’s really serious and we need to urgently do something about this,” she said.

El Baradei, who won the peace prize in 2015, was asked how he would respond to U.S. President Donald Trump’s threat to use unprecedented “fire and fury” against North Korea if it threatened the United States.

“I go to pray,” he said.