The Sunni extremist group now holds 262 Assyrians captive, said Osama Edward, founder of the Assyrian Human Rights Network.
"ISIS is taking over more and more Assyrian towns," he said.
The number has climbed steadily, from an initial estimate of between 70 and 100 people
seized on Monday to 150 as of Wednesday, with women, children and the elderly among them.
The London-based Syrian Observatory for Human Rights put the number of Assyrian hostages seized over three days at 220, in a statement released Thursday.
They were taken from 11 villages in the Tal Tamer area in al-Hasakah Province, the monitoring group said. Its information indicates that ISIS has taken them to the Mount Abdelaziz area, southwest of Tal Tamer.
Edward, who is based in Sweden but has family in the area attacked by the terror group, said Wednesday his information was coming from the Assyrian Human Rights Network's team on the ground.
Edward has said he fears the hostages may face the same fate as as Assyrians targeted in Iraq and the more than 20 members of Egypt's Coptic Christian minority slaughtered by ISIS in Libya last month.
Besides those captured by ISIS militants, thousands of families have been forced to flee their homes, he said.
After years caught up in the middle of a civil war, many of these Assyrians lack food, water, blankets and other basics.
'Burning down churches'
Sharlet and Romel David, in Modesto, California, told CNN affiliate KCRA that 12 of their family members in Syria are believed to be among those kidnapped by ISIS early on Monday, and they fear for their safety.
"We pray, we pray all the time," said Romel.
"What we've heard is it was like a sea of black uniforms marching through all the villages, burning down the churches, desecrating the crosses and wreaking havoc."
Sharlet said her 59-year-old brother had left his job as a car salesman in Modesto two years ago for Syria, to try to bring his son and family back to the United States. Instead, they are thought to be among those held.
The Davids are not naming their relatives for fear it could put them in greater danger.
"I just want them to be safe," said Sharlet.
Kurdish forces fight back
The Assyrians are a proud people who've overcome a lot in their history. They can trace their roots back some 4,000 years to the time of Mesopotamia, considered one of the cradles of civilization and birthplace of writing and literature. While their first religion was Ashurism, Assyrians have been predominantly Christian since the third century.
"How can Syria be Syria without the Assyrians?" Edward said. "We gave the country our name."
ISIS has proven, time and again, its willingness to ruthlessly go after minority groups which don't subscribe to its extreme take on Islam.
Some of their targets -- the Assyrians included -- have taken up arms in an attempt to defend their communities, fighting alongside Kurdish militia, which have made some recent gains against ISIS in Syria's northeast.
But ISIS has faced some opposition.
According to the Syrian Observatory for Human Rights, members of the Kurdish YPG -- or People's Protection Units -- had taken control of 70 villages in al-Hasakah Province as of Wednesday.
They're on the cusp of taking some inhabited by Assyrians and were clashing with ISIS forces around Tal Tamer, the monitoring group reported.