And now, a home for the elderly run by Catholic missionaries is among them.
The Vatican on Saturday reported the killing of four Missionaries of Charity members plus 12 others at an elderly facility in the port city of Aden.
According to an official Vatican News report
, gunmen burst into the building Friday, went room to room handcuffing victims, then shot all 16 in the head.
, an information service for Catholic missionaries, reported the missionaries -- who belonged to the group founded by Mother Teresa
-- were nuns. Two were from Rwanda, one from India and the fourth from Kenya.
The attack -- characterized by Vatican Secretary of State Pietro Parolin as an "act of senseless and diabolical violence" -- spurred a sharp rebuke from Pope Francis.
According to Parolin, the Pope "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."
It wasn't immediately clear who carried out Friday's attack.
Months of violence in poor Arab nation
The impoverished Muslim nation on the southwestern tip of the Arabian Peninsula has faced violence for years, some of it tied to al Qaeda elements that found a home there.
The latest round of unrest began in late 2014 amid angry protests by Houthis, a minority Shiite group that's long held sway in northern Yemen but hadn't had much influence in the country's Sunni-led government.
The Houthi rebels tool over the presidential palace in January 2015, forcing out President Abdu Rabu Mansour Hadi on the way to taking over Sanaa, the capital, and other areas.
Those strikes continue months later, as has the bloodshed.
U.N. Human Rights spokesman Rupert Colville has said
the fighting is taking a "terrible toll" on Yemeni civilians. As of early January, he reported more than 8,100 casualties, including about 2,800 deaths.