Pope Francis says the slaying of four nuns by assassins who handcuffed them before shooting them in Aden, Yemen has "shocked and profoundly saddened" him.

The attackers killed 16 people including the four nuns from the Missionaries of Charity, the order of Mother Teresa, when they attacked an old age home in Aden on March 4.

Reports said the attackers pretended they were visiting their mothers to gain access to the home.

The Vatican said two of the nuns killed were Rwandan, one was Indian and one was from Kenya.

The nuns were working as nurses at the home and had been serving breakfast to its 80 residents when the attack was carried out.

No group has claimed responsibility for the attack, though Yemeni officials have blamed so-called Islamic State.

The Holy See said in its statement, "His Holiness Pope Francis was shocked and profoundly saddened to learn of the killing of four Missionaries of Charity and others at a home for the elderly in Aden.

"He sends the assurance of his prayers for the dead and his spiritual closeness to their families and to all affected from this act of senseless and diabolical violence.

"He prays that this pointless slaughter will awaken consciences, lead to a change of heart, and inspire all parties to lay down their arms and take up the path of dialogue."

A brutal civil war between Iran-backed Zaidi Shia Houthi rebels in the north and the Saudi-Arabia backed government in the south has torn Yemen apart.

The group calling itself Islamic State and Al-Qaeda in the Arabian Peninsula (AQAP) have both been present in the conflict.

The Houthis controlled Aden for months before government loyalists pushed them out in July 2015.

President Abedrabbo Mansour Hadi has declared Aden to be Yemen's temporary capital as Sanaa has been in the hands of the Houthis and their allies since September 2014. But Hadi and many senior officials spend most of their time in Riyadh, Agence France-Presse reports.