Gunmen have stormed a Catholic-run retirement home in Yemen's southern city of Aden, killing 16 people including four Indian nuns.

According to the Associated Press, witnesses said two gunmen remained outside the building while four others entered it. They moved from room to room handcuffing the victims before shooting them in the head.

One nun who survived and was rescued by locals said that she hid inside a fridge in a store room after hearing a Yemeni guard shouting "Run, run."

The home, run by the Missionaries of Charity, an organisation established by Mother Teresa of Calcutta, has about 80 residents. Nuns from the order were attacked in Yemen in 1998, when gunmen killed three of them in the Red Sea port of Hodeida.

Khaled Haidar told The Associated Press that he counted 6 bodies, including that of his brother, Radwan.

He said that in addition to the four Indian nuns, six Ethiopians, one Yemeni cook, and Yemeni guards were among those killed. He said that he spoke to the surviving nun, who was crying and shaking.

Haidar's family took his brother's body for burial; the others were taken to a hospital run by Doctors without Borders or MSF.

Yemen has been in turmoil since the outbreak of a civil war between Iran-backed Shia rebels and the Saudi-backed Sunnis. The south, which has international recognition, is a largely lawless area whose security vacuum is being exploited by Islamic State and al-Qaeda groups.

Once a cosmopolitan city home to thriving Hindu and Christian communities, Aden has gone from one of the world's busiest ports as a key hub of the British empire to a largely lawless backwater.

Its small Christian population left long ago. Unknown assailants have previously vandalized a Christian cemetery, torched a church and last year blew up an abandoned Catholic church.

So far at least 6,200 civilians have died in the conflict and around 2.4 million people have been displaced. There is widespread hunger and the UK is facing calls to stop arms sales to Saudi Arabia amid claims civilians are being killed with UK weapons in the Yemen conflict. The International Development Committee says evidence is "overwhelming" the Saudi-led coalition fighting Houthi rebels has violated humanitarian law.