A suicide bomber attacked a Greek Orthodox Church service in Syria’s capital on Sunday, killing at least 20 people and underscoring the new government’s challenge to maintain stability after more than a decade of civil war.

The Syrian authorities said the assailant had opened fire on congregants at the Mar Elias Church in Dweila, a neighborhood of Damascus, before detonating an explosive vest. More than 50 people were wounded.

The attacker appeared to have ties to the Islamic State, the extremist group that once controlled large areas of Syria, the authorities said. It was the first known suicide bombing in the capital since December, when a rebel coalition ousted Syria’s iron-fisted president, Bashar al-Assad, and took power.

Syria’s new leadership has vowed to restore order and has sought to reassure embattled religious minorities. But sectarian tensions have festered, and for months, the United Nations and U.S. officials have warned that remnants of the Islamic State are seeking to exploit the collapse of the Assad regime to regroup.

The Syrian authorities say they have foiled repeated attack plots in recent months, including some targeting Christian sites.

On Sunday, witnesses to the church bombing described a scene of carnage, with ambulances and local residents rushing to Mar Elias to help. Photos circulated by the state news media showed charred and blood-splattered floors, with shrapnel peppered across the church’s walls.