Pope Francis warned “a good part of humanity” will be destroyed if tensions with North Korea continue to escalate.
He called for diplomacy and a revived United Nations to take the lead in negotiations as the US warned of “catastrophic” consequences if the rogue state is not stopped.
His announcement came just hours after the Pyongyang government reportedly fired another failed ballistic missile from Pukchang Airfield on Thursday.
US President Donald Trump, who previously said the United States could “absolutely” go to war over Kim Jong-un’s nuclear threat, has sent an American carrier to conduct drills near the Koreas.
Asked what he would tell Mr Trump in the face of the crisis, Francis said he would urge him to use diplomacy and negotiation “because it’s the future of humanity.”
He said "Today, a wider war will destroy not a small part of humanity, but a good part of humanity and culture," adding, "it would be terrible. I don't think humanity today could bear it."
On Friday, the UN Security Council held a ministerial meeting on Pyongyang's escalating weapons programme.
North Korean officials boycotted the meeting, which was chaired by US Secretary of State Rex Tillerson.
North Korean ballistic missile tests are banned by the United Nations because they are seen as part of the North's push for a nuclear-tipped missile that can hit the US mainland.
Francis said the UN should regain its leadership in conflict resolution, saying it had been "watered down" over time.
Mr Trump has sent a nuclear-powered submarine and the USS Carl Vinson aircraft supercarrier to Korean waters.
The rogue state this week conducted large-scale, live-fire exercises on its eastern coast.
The US and South Korea also started installing a missile defence system that is supposed to be partially operational within days, and their navies began joint military drills on Saturday.
The White House has said it would reach out to the Vatican to arrange an audience when Mr Trump travels to Sicily at the end of May for a G7 summit.