"Nineteen Assyrian hostages arrived on Sunday at the Church of Our Lady in Hasakeh after they were released by IS," said Osama Edward, the director of the Assyrian Network for Human Rights.
"They arrived on two buses from Shaddadeh," the IS stronghold in the northeastern province of Hasakeh where they had been detained, he told AFP.
Edward said an IS religious court decided on Saturday to release the Christians in exchange for a sum of money for each family that IS considers as jizya, or tax, paid by non-Muslims.
The activist said negotiations for the release of all hostages began on Saturday between Assyrian officials and Arab Muslim tribal chiefs.
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Last week, IS kidnapped 220 Assyrians in the Tal Tamr area where the extremist Islamist group has seized control of 10 Christian villages, said the Syrian Observatory for Human Rights.
Almost 5,000 people have since fled to Kurdish- and government-controlled areas.
Before Syria's civil war erupted in 2011, there were 30,000 Assyrians in the country, among an estimated Christian population of about 1.2 million.
UN peace envoy Staffan de Mistura paid a surprise visit on Sunday to a church near the Syrian capital in a show of solidarity with the war-torn country's Christian minority targeted by jihadists.