The abductions took place on Tuesday after IS seized two Assyrian villages from Kurdish forces in the province of Hassakeh, according to the Syrian Observatory for Human Rights.
The Britain-based monitor had no details on the missing Assyrians, who were taken from two villages — Tal Shamiram and Tal Hermuz — after they were attacked by IS.
IS has destroyed churches and Christian shrines in Syria, and demanded that Christians living under its rule pay a tax known as jizya.
In Libya, IS jihadists last week released a video showing the beheadings of 21 mostly Egyptian Christians.
Much of Hassakeh is divided between Kurdish and IS control.
Fighters from the Kurdish People’s Protection Units (YPG) have been on the offensive in the province in recent days.
They have taken 24 villages and hamlets as part of an operation to try to recapture the town of Tal Hamis and surrounding areas.
Tal Hamis lies to the east of the villages taken by IS on Tuesday.
YPG forces have also been on the offensive in Raqa province, which neighbours Hassakeh, seizing 19 villages as they advance following their recapture of the strategic border town of Kobane last month.
The Kurdish forces have been backed by US-led air strikes launched by the international coalition fighting IS.
The Observatory said the coalition carried out a series of strikes around Tal Hamis on Tuesday that killed 14 IS members.