Pope Francis on Sunday called for the release of hostages, including six nuns, who were kidnapped on a bus in Haiti on Friday, and said he was praying for social harmony in the country.

Armed gunmen hijacked a bus in Haiti's capital Port-au-Prince with at least six nuns on board and drove off to an unknown destination taking all passengers hostage, Vatican News reported on Saturday, citing the Haitian Conference of Religious group.

"I have learned with sorrow of the kidnapping, in Haiti, of a group of people, including six religious sisters", Pope Francis said after his weekly Angelus prayer. 

"In my heartfelt plea for their release, I pray for social concord in the country, and I invite everyone to bring an end to the violence, which is causing a great deal of suffering to that dear population," he added. 

Violence has been escalating for months in Haiti as gangs jockey for power and seek to put pressure on interim Prime Minister Ariel Henry, ahead of the Feb. 7 expiry of a political accord that consolidated his power.

The violence comes ahead of a court decision expected on Jan. 26 on a Kenyan-led multinational force to address gang violence in the country, one of the poorest in the Western Hemisphere.