AL MUKALLA, Yemen — A nursing home founded by Mother Teresa and run by Christian nuns in the southern Yemeni city of Aden was transformed into an execution ground on Friday, when four gunmen tied up at least 16 people inside and killed them, officials said.

The victims were Indian, Ethiopian and Yemeni citizens, and they included six nuns, plus guards and the gardener, said Ayoub Abu Baker, the director of the local Ministry of Social Affairs and Labor, which runs the nursing home along with a group called the Missionaries of Charity.

The 60 residents of the nursing home were not harmed, officials said.

Aden has been battered by expanding violence over the last few months as a result of the war that has engulfed the country, the Arab world’s poorest, pitting the Houthi rebel movement against a Saudi-backed Yemeni government. Local armed groups have been competing for power and Sunni extremists belonging to the Islamic State and Al Qaeda have carved out a presence, punctuated by assassinations and larger-scale deadly attacks.

But even amid the growing mayhem, the killings on Friday provoked outrage and shock, with pictures of the prone bodies of the nursing home workers circulating on social media.

In a statement, Yemen’s United Nations mission called the slaughter a “cruel and heartless act” and suggested that Islamic State militants had been responsible, asserting that the perpetrators possessed “nothing of humanity or Islam.”

There was no immediate claim of responsibility, but the killings recalled other recent attacks on Christian sites in Aden, which was known in previous eras for its tolerance and mix of religious faiths.

The city’s residents now face episodic warfare and increasingly capricious attacks. Forces allied with the Saudi-backed government of President Abdu Rabbu Mansour Hadi took control of the city last summer, driving out the Houthis, who control much of northern Yemen.

But the government has not been remotely successful in pacifying the city — or even protecting its own officials, with Mr. Hadi’s presidential palace coming under repeated attack.

Local media accounts of the nursing home massacre said the gunmen might have posed as visitors to gain entry. Mr. Abu Baker said the assailants had isolated their victims in a room and bound their hands before killing them.

The nursing home was one of several care facilities in Yemen established by Mother Teresa beginning in the 1970s, according to an article about her work in Yemen posted on the website of the Indian Embassy.

Mr. Abu Baker said that other nursing homes were operating in at least three other cities around Yemen.

News of the killings came as United Nations human rights officials warned of the sharply escalating civilian death toll from airstrikes by the Saudi-led coalition in northern Yemen.

The United Nations recorded a total of 168 civilians killed and 193 injured in February, double the number of the previous month and the highest monthly casualty toll since September, said Rupert Colville, a spokesman for the Office of the High Commissioner for Human Rights in Geneva.

Two-thirds of the casualties came from coalition airstrikes, mostly in Sana, the capital, Mr. Colville said, adding that one air attack on a market in the city’s northeast on Saturday had killed at least 39 civilians, including nine children. An airstrike on a cement factory in Amran, northwest of the capital, killed 14 civilians and injured 39, he said.

The human rights office said indiscriminate shelling by Houthi forces and army units loyal to the former president, Ali Abdullah Saleh, had left 49 civilians dead in February, mostly in the fiercely contested city of Taiz.

The latest casualty figures raised the number of civilian deaths documented by the United Nations since the Yemen conflict began a year ago to 3,081, with 5,733 injured.

Increasingly angered over the disproportionately high civilian casualties caused by the Saudi-led bombings, the European Parliament recently voted by a large majority for an embargo on weapons exports to Saudi Arabia.

The top humanitarian aid official of the United Nations, Stephen O’Brien, told the Security Council on Thursday that all parties to the conflict had attacked civilian infrastructure, including hospitals and schools.

The Security Council is weighing a possible resolution to push for more emergency aid to Yemen, which may be circulated to members as early as next week. But the Saudis signaled on Friday that they opposed such a measure.

“We don’t think that a resolution is needed at this time,” the Saudi ambassador to the United Nations, Abdallah al-Mouallimi, said at a news conference.