Gunmen who killed at least 15 people in an old people's home in Yemen last week also kidnapped an Indian priest, officials said on Sunday, as Pope Francis condemned the attack and the "indifference" of the world's reaction to it.

No one has claimed responsibility for Friday's incident in which four gunmen posing as relatives of one of the residents at the home burst inside, killing four Indian nuns, two Yemeni female staff members, eight elderly residents and a guard.

Indian Foreign Minister Sushma Swaraj said on Twitter that an Indian national identified as Father Tom Uzhunnalil had been "abducted by terrorists in Yemen". She said officials in neighbouring Djibouti were trying to ascertain his whereabouts to secure his release.

Officials in the southern Yemeni city of Aden confirmed that the priest had been kidnapped and said authorities were investigating the attack. It sparked widespread condemnation, including from the Pope and the government of President Abd-Rabbu Mansour Hadi, which called it an act of terrorism.

Pope Francis called the nuns "today's martyrs" because they were both victims of their killers and of global indifference.

"They do not make the front pages of the newspapers, they do not make the news. They have given their blood for the Church," he said in his Sunday message to thousands of people in St Peter's Square.

"They are victims of the attack by those who killed them but also victims of indifference, of this globalisation of indifference. They don't matter," he added, departing from his prepared text.

International aid groups have pulled most of their foreign staff from Yemen but continue to operate on a reduced basis through local employees.

Aden has been racked by lawlessness since Hadi supporters, backed by Gulf Arab military forces, drove fighters of the Iran-allied Houthi group from the city in July last year.

The Yemeni government has repeatedly promised to restore security to the city but has so far had little success.