Pope Francis says the four nuns killed in an attack on a home for the elderly in Yemen are modern-day martyrs and victims of indifference.

Francis told the faithful Sunday that the nuns "gave their blood for the Church" and that they were not only victims of the attackers but also of "this indifference of globalization."

The nuns were among 16 people killed Friday by gunmen who stormed a retirement home run by a charity established by Mother Teresa.

The pope also praised an ecumenical project to fly refugees to Europe as "a concrete sign of the commitment for peace and life." Around 100 refugees arrived in Rome last month, the first wave of the planned transfer of 1,000 particularly vulnerable refugees from camps in Lebanon, Morocco and Ethiopia.