AL MUKALLA, Yemen — A nursing home founded by Mother Teresa and run by Christian nuns in the southern Yemeni city of Aden was transformed into an execution ground on Friday, when four gunmen tied up at least 16 people inside and killed them, officials said.

The victims were Indian, Ethiopian and Yemeni citizens, and they included six nuns, plus guards and the gardener, said Ayoub Abu Baker, the director of the local Ministry of Social Affairs and Labor, which runs the nursing home along with a group called the Missionaries of Charity.

The 60 residents of the nursing home were not harmed, officials said.

Aden has been battered by expanding violence over the last few months as a result of the war that has engulfed the country, the Arab world’s poorest, pitting the Houthi rebel movement against a Saudi-backed Yemeni government. Local armed groups have been competing for power and Sunni extremists belonging to the Islamic State and Al Qaeda have carved out a presence, punctuated by assassinations and larger-scale deadly attacks.

But even amid the growing mayhem, the killings on Friday provoked outrage and shock, with pictures of the prone bodies of the nursing home workers circulating on social media.