ASSISI, Italy - Pope Francis said Tuesday the world "thirsts for peace" after praying with faith leaders for an end to religious fanaticism and indifference to the plight of war victims.

"We do not have weapons" to end wars and stop those who commit violence in the name of God, the Argentine pontiff told religious heads from across the globe gathered in the central Italian medieval town of Assisi.

But "as religious leaders, (we) are duty bound to be strong bridges of dialogue, creative mediators of peace," he said, as fighting resumed in Syria and several people died in a second day of bloody violence in DR Congo.

"We never tire of repeating that the name of God cannot be used to justify violence," and must cast off "the heavy burdens of distrust, fundamentalism and hate," he told an open-air ceremony as the sun set.

The annual World Day of Prayer event, established by John Paul II 30 years ago and held in the hill-side town, aims to combat extremism in the name of religion and the persecution of people for their faiths.

The pope had held one-on-one talks with leaders including the Archbishop of Canterbury Justin Welby, the Orthodox Patriarch of Constantinople Bartholomew I and Din Syamsuddin, chairman of Muhammadiyah organisation.

He also sat down with Rabbi David Rosen from the American Jewish Committee and Koei Morikawa, the Supreme Head of the Tendai Japanese school of Mahayana Buddhism. The 79-year-old pontiff had arrived amid tight security for lunch with the leaders and a group of refugees, including Syrians and Palestinians, an Eritrean, two Nigerian women and a 23-year-old from Mali who fled crisis-torn Libya for Sicily by boat.

The leaders had prayed separately in locations across the town before coming together in the square in front of the Basilica of Saint Francis to read out messages of peace.

Twenty-seven candles were lit for 27 war-hit countries, from Iraq to Myanmar, Syria and Yemen, before a minute's silence.

"Islam is a religion of peace," Syamsuddin said, adding that it was "the responsibility of us Muslims to work together to show the real face of our faith" and distance it from those who perpetrate violence in the name of Islam.

In his address, Buddhist leader Morikawa bemoaned military interventions aimed at ending conflicts, saying history had taught us "peace enforced will be undone through force".

Bartholomew called for fundamentalists in each religion to be "isolated" while Israeli Rabbi David Brodman, who survived the Holocaust as a child, said the Assisi meeting was proof "even those who are different can forge friendships and live in peace".

After hearing testimony from a Syrian woman who described the screams of children under fire in war-torn Aleppo, Francis said the world needed to give voice to those who suffer. "They know well, often better than the powerful, that there is no tomorrow in war," he said.

The pontiff had earlier reminded Western countries that while they had suffered a string of deadly militant attacks, there were parts of the world where cities were being flattened by fighting, prisoners were being tortured and families were starving to death.

"We are frightened... by some terrorist acts," he said as US authorities probed an attack possibly linked to the Islamic State group. "(But) this is nothing compared to what is happening in those countries, in those lands where day and night bombs fall."