Syrian interim President Ahmed al-Sharaa vowed Monday that those involved in a "heinous" suicide attack on a Damascus church a day earlier would face justice, calling for unity in the country.

The shooting and suicide bombing Sunday at the church in the working-class Dwelaa district of the Syrian capital killed 25 people and wounded 63, the health ministry said, raising an earlier toll of 22 killed.

The authorities said the attacker was affiliated with the Islamic State group.

"We promise... that we will work night and day, mobilising all our specialized security agencies, to capture all those who participated in and planned this heinous crime and to bring them to justice," Sharaa said in a statement, AFP reported.

The attack "reminds us of the importance of solidarity and unity of the government and the people in facing all that threatens our nation's security and stability", he added.

Condemnation has continued to pour in from the international community after the attack -- the first of its kind in the Syrian capital since the toppling of longtime ruler Bashar al-Assad in December.

It was also the first inside a church in Syria since the country's civil war erupted in 2011, according to a monitor, in a country where security remains one of the new authorities' greatest challenges.

Since the new authorities took power, the international community has repeatedly urged the government to protect minorities and ensure their participation in Syria's transition, particularly after sectarian violence in recent months.