They were seized when the terrorists swept through al-Qaryatain in Homs province, monitoring groups say.
Many of the Christians had fled to al-Qaryatain to escape fighting in Aleppo province to the north.
ISIS has treated Christians harshly in other places under its control.The group follows its own extreme version of Sunni Islam and has previously ordered Christians to convert, pay jizya (a religious levy), or face death.
According to the UK-based Syrian Observatory for Human Rights (SOHR), at least 230 people were seized by ISIS fighters in al-Qaryatain.
Some were taken from inside a church, SOHR said. It said ISIS was hunting down a list of people it suspected of "collaborating with the Syrian government ".
The Assyrian Federation of Sweden, whose members have relatives in the town, said about 100 families were being held.
The federation says it has not been able to contact anyone inside the town and the conditions of the captives are not known.
The SOHR said "at least 60" Christians were among those being held.
Earlier this year, dozens of Assyrian Christians were abducted by ISIS militants in attacks on villages in Hassakeh province in north-east Syria.
A local Christian militia said it believed the captives were taken away to a mountain, but their fate since then is unclear.