Leo, who has drawn the ire of US President Donald Trump after criticising the Iran war, asked worshippers on Friday to pray that global governments would turn away from violence.
In a visit to Pompei, a modern city about 245 km south of Rome near the famed ruins of a volcanic eruption, the pope said he would join their prayers that God would begin "touching hearts, calming rancour and fratricidal hatreds, and enlightening those who have special responsibilities of government".
Leo, the first US pope, held talks with Rubio on Thursday in an atmosphere of tension with Washington as Trump has repeatedly disparaged the pontiff on social media.
The Vatican said afterwards that the two had pledged to improve their bilateral relations, in what insiders said was an unusual recognition of unprecedented tensions.
The US embassy to the Holy See said on X after the meeting that Leo and Rubio had discussed "topics of mutual interest in the Western Hemisphere".
Leo, the former Cardinal Robert Prevost, was elected by the world's cardinals to succeed the late Pope Francis as leader of the 1.4-billion-member Church on May 8, 2025.
Prevost, who spent decades as a missionary and a bishop in Peru before becoming pope, kept a relatively low profile in his first 10 months but has been speaking forcefully against war and despotism in recent weeks.
In his message to thousands in Pompei's main square on Friday, the pontiff lamented that world peace is "endangered by international tensions and by an economy that prefers the arms trade to respect for human life".
He urged people not to become accustomed to war.
"We cannot resign ourselves to the images of death that the news shows us every day," said Leo.
In a later visit on Friday to Naples, Italy's third-largest city, the pope praised initiatives by groups there to welcome refugees coming to Italy from Gaza.
Leo urged Neapolitans to "continue to give voice ... to a culture of peace, countering the logic of confrontation and the force of arms as the presumed solution to conflicts."
