Gunmen killed at least 16 people in an attack on a Catholic retirement home in Yemen's port city of Aden that was established by Mother Teresa.
According to one official, the assailants entered the premises in Aden's Sheikh Othman district after telling the guard they were visiting their mother, before storming the building and opening fire.
Friday's casualty figures include four Catholic nuns from India, four local nurses, four security guards, and three cleaning staff, medical sources told Al Jazeera.
"They forced the men and women outside with their hands tied. We heard the sound of gunfire, and when we came out we saw them all dead in the garden," resident Um Mohammed said.
Screams of elderly residents echoed from the home during the shooting rampage, witnesses told they AFP news agency, adding they saw bodies of dead workers with their arms tied behind their backs scattered on the floor.
The motive of the gunmen, who fled after the attack, was not immediately known. No group has yet claimed responsibility.
Pope Francis on Saturday described the killlings as "diabolical".
"His Holiness Pope Francis was shocked and profoundly saddened to learn of the killing of four Missionaries of Charity and 12 others at a home for the elderly in Aden," the Vatican's Secretary of State Pietro Parolin said.
The Argentine pontiff "prays that this pointless slaughter will awaken consciences, lead to a change of heart, and inspire all parties to lay down their arms and take up the path of dialogue", Parolin said.
Aden was once a cosmopolitan city home to thriving Hindu and Christian communities but its small Christian population left long ago.
Unknown assailants have previously vandalised a Christian cemetery, torched a church and last year blew up an abandoned Catholic church.
READ MORE: 'Humanitarian catastrophe' unfolding in Yemen, says UN
Yemen descended into a civil war in March when the Houthi fighters forced President Abd-Rabbu Mansour Hadi to flee to Saudi Arabia after they closed in on Aden, drawing in an Arab coalition assembled by the Saudis into the conflict.
The United Nations says nearly 6,000 people have been killed in the fighting, while hundreds of thousands have been displaced from their homes.
Source: Al Jazeera and agencies