Caritas, the social development arm of the church, urged the government and Tiger rebels to honor their 2002 ceasefire agreement amid a surge in violence that has seen 815 deaths since December.
The assassination of Major General Parami Kulatunga, the army's number three, was the latest blow to the unravelling peace process and brought Sri Lanka closer to full-scale war.
"Our fervent plea to leaders in Sri Lanka is to stick strictly to the ceasefire agreement, stop the violence and destruction of human beings and return to the negotiating table," said Caritas secretary general Duncan MacLaren.
More than 60,000 people have died during three decades of ethnic conflict in Sri Lanka.
Delegates from Sri Lanka, Colombia, the Democratic Republic of Congo, Ireland and the Philippines were meeting along with Palestinian representatives for three days to share their experiences in peace-building.
"The arrival of all these delegates, many of whom have huge experience in peace-brokering in confli