At least 20 people were killed and dozens injured when a suicide bomber blew himself up at the Mar Elias Church in the Dweila neighbourhood of Syria's capital, Damascus, on Sunday, health authorities and security sources said.

It was the first suicide bombing in Damascus since Bashar al-Assad was toppled by an Islamist-led rebel insurgency in December.

Syria's interior ministry said the suicide bomber was a member of the Islamic State. He entered the church, opened fire and then detonated his explosive vest, a ministry statement added.

A security source, speaking on condition of anonymity, said two men were involved in the attack, including the one who blew himself up.

Islamic State has been behind several attempted attacks on churches in Syria since Assad's fall, but this was the first to succeed, another security source told Reuters.