Gunmen in southern Yemen on Friday stormed a retirement home run by a charity established by Mother Teresa, killing 16 people, including four Catholic nuns, officials and witnesses said.

The killing spree began with two gunmen who first surrounded the home for the elderly in Aden. Meanwhile, four others entered the building on the pretext they wanted to visit their mothers at the facility, according to the charity, Yemeni security officials and witnesses. The officials spoke on condition of anonymity because they were not authorized to speak to the media.

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

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

He said that 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.

Sunita Kumar, a spokeswoman for the Missionaries of Charity in the Indian city of Kolkata, said the members of the charity were "absolutely stunned" at the killing.

"The Sisters were to come back but they opted to stay on to serve people" in Yemen, she added.

She also said that two of the killed nuns were from Rwanda and the other two were from India and Kenya.

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.

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 were around 80 residents living at the home, which is run by Missionaries of Charity, an organization established by Mother Teresa. 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.

Gunmen attacked a retirement home in Aden, Yemen, officials said Friday. (CBC/Google)

Aden descended into lawlessness after a Saudi-led coalition recaptured the city from Shia Houthi rebels last summer.

Yemen's civil war has split the country in two. The northern region, where Shia 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 group and Yemen's al-Qaeda affiliate have exploited the lawlessness and created safe havens in the south. Al-Qaeda controls several southern cities while IS 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.