They were seized when the jihadists 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.
Islamic State (IS) 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 Islamic State (IS) fighters in al-Qaryatain.
Some were taken from inside a church, SOHR said. It said IS was hunting down a list of people it suspected of "collaborating with the regime".
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 IS 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.
Syria's beleaguered Christians

Richie, the elder at 27 and taller at 6ft 9in, with 63 caps and a 2013 Lions Test on his CV, has been a mainstay for Scotland since his debut in 2010.
But the recent progress of 6ft 6in Jonny, 23 next week, has seen him earn most plaudits as a potential 2017 Lion.
"Jonny has gone from strength to strength," Richie Gray acknowledged.
"What I said a couple of years ago was about the need to do it consistently, but he's been able to do it week in week out and that's the most impressive thing.
"I've seen him come on as a leader. He's grown and matured and makes me very proud.
"How has his emergence as a leader changed our relationship? Not too much. We still have our discussions about things and what we can get better at.
"We're not really competitive. I think he wins most weeks anyway. It's not much of a competition.
"With brothers I suppose you always want to win the battle but we're not too much like that. We try and help each other out as much as possible.
"I see him more as an inspiration. It's great to see and certainly keeps me going. It gives me a standard I have to aim for. I'm just trying to keep up."
Jonny Gray, a vice-captain to stand-in skipper John Barclay, is the leading tackler in this year's Six Nations, with 52 from 54 attempts. He is also in the top 10 for carries, with 36 in the three rounds to date.
Richie, meanwhile, has made more tackles than anyone else without missing one (41) so far.
They are both likely to make plenty more on Saturday if Scotland are to achieve a first win at Twickenham for 34 years and secure a first Triple Crown since 1990.
But even if victory eludes them, the elder Gray believes a bright future lies ahead for the class of 2017.
"It's a great position to be in," said the Toulouse lock. "Obviously it's a huge challenge going down there and a match with a huge amount of importance. We're looking forward to it.
"We've constantly been progressing with the group of players and the staff. We have been getting better and better.
"Glasgow and Edinburgh have both been doing well in Europe and, with us having three home games this year, hopefully that all adds together to form a good championship.
"But there certainly is a lot more to come. It's a young group and we have got more things to show."
Islamic State militants have abducted dozens of people, many Christian, from a Syrian town captured on Thursday from pro-regime forces, reports say.
[NEXT_CONCEPT]
Scotland lock Richie Gray says younger brother Jonny is an "inspiration" rather than a sibling rival in their burgeoning second-row partnership.