Osama Edward, director of the Christian Assyrian Network for Human Rights in Syria, told The Associated Press that about 60 Christians were initially taken but that about half of them were released and had made it to nearby villages.
Qaryatain lies in the middle of a triangle formed by the cities of Homs, Palmyra and Damascus. Activists say it has a mixed population of around 40,000 Sunni Muslims and Christians, as well as thousands of internally displaced people who had fled from Homs.
The Observatory said that "dozens" of Christian were among the people abducted, while al-Talawy put the number at 60. He said the rest of those abducted, about 170 people, are Sunni Muslims.
The Observatory also said there were 45 women and 19 children among those abducted. Assyrian Christian priest Gabriel Dawoud told the Lebanon-based Al-Mayadeen TV that two of abducted Christians were in wheelchairs.
Edward, the Assyrian activist, said IS appears to be preparing to attack the Christian town of Sadad, currently home to more than 4,000 Christian families, that was captured by al-Qaida's fighters in 2013 but was retaken by Syrian troop shortly afterward.
Many of the Sadad townspeople are already fleeing the town, he said.
Amnesty International condemned the abductions. Neil Sammonds, the London-based group's Syria researcher, said the abductions were "abhorrent" and highlighted the "dreadful plight of civilians caught up in the conflict in the country."
"Every effort must be made to identify the perpetrators of these crimes and bring them to justice," Sammonds added.
The Turkey-based Syrian National Coalition, the main Western-backed opposition group, claimed that President Bashar Assad's government facilitated the capture of the religiously mixed Qaryatain "in order to exploit the matter politically."
Christians make up about 10 percent of Syria's prewar population of 23 million.
In February, IS kidnapped more than 220 Assyrian Christians, after overrunning several farming communities on the southern bank of the Khabur River in the northeastern province of Hassakeh.
Since then, only a few have been released and the fate of the others remains unknown.
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