A team of gunmen unleashed a massacre at a retirement home run by Catholic nuns in Yemen Friday, killing 16 people including four nuns, Yemeni security officials and witnesses said.

The gunmen then moved from room to room, handcuffing the victims before shooting each of them in the head. A nun who survived said that she hid inside a fridge in a storeroom after hearing a Yemeni guard shouting "run, run."

Missionaries of Charity, an organization established by Mother Teresa, runs the home in the chaotic southern port city of Aden, which descended into lawlessness after a Saudi-led coalition recaptured the city from Shiite Houthi rebels last summer.

Yemen's civil war has split the country in two. The northern region, where Shiite rebels are in control, has been struck by an extensive air campaign by a Saudi-led coalition. The southern region, which is controlled by the internationally-recognized government backed by Saudi Arabia, is suffering from a power and security vacuum. The Islamic State terror group and al-Qaida affiliates have exploited the lawlessness and created safe havens in the south.

No terror group immediately claimed credit for Friday's slaughter.

Khaled Haidar told The Associated Press that he counted 16 bodies including that of his brother, Radwan, in the home. All had been shot in the head and were handcuffed. He said that in addition to the four nuns, six Ethiopians, one Yemeni cook, and Yemeni guards were among those killed.

Haidar said his family was the first to arrive at the house and that he spoke to the surviving nun, who was crying and shaking. Haidar said that his family later handed her over to a group of southern fighters in charge of security in the local Aden district of Sheikh Osman.

Vikas Swarup, the spokesman of India's External Affairs Ministry, said the attackers had asked the guard to open the gate on the pretext of visiting their mothers at the retirement home.

"On entering inside, (they) immediately shot dead the gatekeeper and started shooting randomly," he said, adding that the assailants escaped soon after the attack.

The bodies were transferred to a police station and then a hospital run by the aid organization known as Doctors Without Borders or MSF. An official with MSF confirmed that 15 bodies had arrived at the hospital. Haider said his family took his brother's body for burial.

There are around 80 residents living at the home. Missionaries of Charity nuns also came under attack in Yemen in 1998, when gunmen killed three nuns in the Red Sea port city of Hodeida.

Al-Qaida controls several southern cities and ISIS has claimed responsibility for a wave of deadly attacks in Aden, including a suicide bombing that killed the city's governor and several assassination attempts on top officials.

Aden's churches have also come under attack. In the summer, a Catholic church in the district of Crater was torched and sabotaged by Islamic extremists.

Yemen's war has killed at least 6,200 civilians and injured tens of thousands of Yemenis, and 2.4 million people have been displaced.

Two of the killed nuns were from Rwanda and the other two were from India and Kenya, Missionaries of Charity spokeswoman Sunita Kumar said. Earlier, Yemeni and Indian officials reported that all four killed nuns were Indian but such conflicting information on casualties is not unusual in the chaos of Yemen's civil war. India's foreign ministry had initially cited information it got from its embassy in Yemen.

The Associated Press contributed to this report.