Pope Francis has warned that international relations can no longer be "held captive" by policies of fear and nuclear deterrence, as he urged the world to get behind his vision of a future free from atomic weapons.
Francis addressed Nobel peace laureates, UN and Nato officials and diplomats from countries with the bomb during a Vatican conference aimed at galvanising support for a global shift from the Cold War-era policy of nuclear deterrence to one of disarmament.
Speaking in the Clementine Hall of the Apostolic Palace, Francis acknowledged that current tensions might render efforts at ridding the world of nuclear weapons remote.
But he said reliance on such weapons "create nothing but a false sense of security", and that any use of them, even accidental, would be "catastrophic" for humanity and the environment.
He said: "International relations cannot be held captive to military force, mutual intimidation, and the parading of stockpiles of arms."
Francis added that "progress that is both effective and inclusive can achieve the utopia of a world free of deadly instruments of aggression".
He also endorsed a new UN treaty calling for the elimination of atomic weapons, saying it filled an important gap in international law.
Beatrice Fihn, executive director of the International Campaign to Abolish Nuclear Weapons (Ican), the advocacy group that won this year's Nobel Peace Prize for its instrumental role in getting the treaty passed, is among the speakers at the two-day Vatican meeting.
The conference comes amid mounting tensions on the Korean peninsula and heated rhetoric between Washington and Pyongyang over the North's nuclear ambitions.
The event's organiser, Cardinal Peter Turkson, told participants that the gathering was planned well before US president Donald Trump began his current trip to Asia, where the Korean nuclear threat has topped his agenda.
Drawing laughs from the largely secular audience, Cardinal Turkson said it was "divine providence" that the conference and US president's trip coincided.
The conference is the first major international gathering since 122 countries approved the UN nuclear weapons treaty in July.
None of the nuclear powers or Nato members signed on to the accord, arguing that its lofty ideals were unrealistic given the rapid expansion of North Korea's nuclear weapons programme and other nuclear threats.
Ms Fihn, head of the Nobel-winning Ican, said the treaty will have an impact even on the nuclear-armed countries which refused to participate.
Previous treaties banning chemical and biological weapons were a crucial first step in making such arsenals illegal, and put pressure on countries which had the weapons to disarm, she said.
"If international law says it's prohibited, it's going to make it a lot harder for them (nuclear states) to justify their decisions to modernise and invest in new types of weapons," she said.
Francois Bugnion of the International Committee of the Red Cross warned that if nuclear weapons were to be used, the effects would be devastating for humanity and future generations.
"As the (Red Cross) learned in Hiroshima, there are no effective means of assisting survivors while protecting those delivering assistance," Mr Bugnion said.
"The majority of victims will be denied the medical assistance they need."
The United States was represented at the conference by its deputy ambassador to the Holy See, Luis Bono, while Russia sent an ambassador and a top nuclear expert, Alexei Arbatov.
China and North Korea were invited, but organisers said they did not attend. Neither has diplomatic relations with the Holy See.
Mr Bono said he wanted to be there because "we're interested to hear what the Holy See is saying" about nuclear disarmament.
He noted that Mr Trump was in China meeting with President Xi Jinping and trying to find ways to pressure North Korea to abandon its nuclear aspirations.
In his speech, Francis did not mention North Korea by name. The Vatican has ruled out - at least publicly - assuming a mediation role in the tense dispute.
However, Cardinal Turkson told reporters the Vatican was seeking direct contact with the North via the bishops' conference of South Korea.