Dozens of Assyrian Christians were abducted from villages in north-eastern Syria in February.
The head of a Syrian Assyrian group in Sweden confirmed the release to Reuters and said all of the freed captives were elderly men and women.
In total 22 were released but it's thought they continue to hold more than 150 Christians taken at the same time.
More than 200 Christians were captured by the group when it overran the Hasaka region.
The head of a Syrian Assyrian group in Sweden, Afram Yakoub, told Reuters: "Some have health issues, so we believe they released them because of health issues and because they are old.
"The release is a small sign of hope. It gives us some hope that someday those remaining will be released."
It's not known is a ransom was paid.
Syrian priest Revd Nadim Nassar told Premier's News Hour it wasn't a sign the terrorists were becoming more compassionate.
"What compassion?," he questioned. "After beheading thousands of people? So because they released 20 Christians it means they are showing compassion?"
He added: "It could be a political deal, an economic deal so that they released them and they got money from the other hand.
"I don't see any compassion, I see monsters leashing their evil in my country."
Revd Nadim Nassar speaking to Antony Bushfield on Premier's News Hour:
