Story highlights

NEW: Pope prays publicly for victims of Yemen attack

Gunmen handcuffed victims, then shot each one in the head

Al Qaeda: "This is not ... our way of fight"

CNN  — 

There are few places in Yemen – homes, schools, hospitals – that haven’t been scarred by the ongoing war ravaging that Arab nation.

Gunmen raided the building on Friday, handcuffing victims and shooting each in the head.

The attack at the facility run by Catholic missionaries in the port city of Aden left four nuns dead, the Vatican reported.

“These are the martyrs of today,” Francis said Sunday in St. Peter’s Square, calling the nuns not just victims of the attackers but also of “indifference, of this globalization of indifference, that does not care.”

The nuns were part of a group founded by soon-to-be-sainted Mother Teresa. Two were from Rwanda, one was from India and the fourth one was from Kenya.

In his prayer, Francis called on Mother Teresa to “accompany in heaven these daughters, martyrs for charity, and intercede for peace and sacred respect for human life.”

Al Qaeda militant group: ‘It’s not our way of fight’

Ansar al-Sharia, an umbrella group for al Qaeda militants in Yemen, said it is not responsible for the attacks. It warned journalists to avoid reporting that it is responsible.

“Our honorable people of Aden, we Ansar al-Sharia deny any connection or relation to the operation that targeted the elders’ house,” the group said in a statement Sunday. “This is not our operation and it’s not our way of fight.”

It was not immediately clear who carried out the attack.

The unusual statement stands in a sharp contrast with previous responses from al Qaeda and other terror groups, which have proudly claimed responsibility for other attacks.

Vatican condemns attackers

On Saturday the Vatican sharply rebuked the gunmen and urged a resolution.

The Pope “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,” Vatican Secretary of State Pietro Parolin said.

It wasn’t immediately clear who carried out Friday’s attack.

Months of violence in poor Arab nation

The impoverished Muslim nation on the southwestern tip of the Arabian Peninsula has faced violence for years, some of it tied to al Qaeda elements that found a home there.

The latest round of unrest began in late 2014 amid angry protests by Houthis, a minority Shiite group that’s long held sway in northern Yemen but hadn’t had much influence in the country’s Sunni-led government.

The Houthi rebels tool over the presidential palace in January 2015, forcing out President Abdu Rabu Mansour Hadi on the way to taking over Sanaa, the capital, and other areas.

But Hadi would return in force, aided by warplanes from Saudi Arabia, the United Arab Emirates, Kuwait, Bahrain, Qatar and others that began going after Houthi targets last March.

Those strikes continue months later, as has the bloodshed.

U.N. Human Rights spokesman Rupert Colville has said the fighting is taking a “terrible toll” on Yemeni civilians. As of early January, he reported more than 8,100 casualties, including about 2,800 deaths.

In January, he reported over 8,100 casualties, including 2,800 deaths. That number is expected to go up when new numbers are released.

CNN’s Greg Botelho, Schams Elwazer and Yousuf Basil contributed to this report.