Osama Edward, founder of the Assyrian Human Rights Network, told CNN that the extremist group is now believed to have 150 hostages, citing the latest information from the network's team on the ground.
The video message will be directed to President Barack Obama and other members of the U.S.-led coalition against ISIS, Edward said from Stockholm, Sweden.
The Assyrians were kidnapped early Monday from villages near Tal Tamer in northeastern Syria.
Edward, who has family in the area attacked by the terror group, said the latest information from the ground indicated the hostages had been moved to an ISIS-controlled location.
The activist organization Raqqa Is Being Slaughtered Silently also said on Twitter that ISIS has moved a number of Assyrian captives to Raqqa, which has been called the Islamist extremist group's de facto capital.
The last message received from the hostages was a text from a kidnapped woman to her husband, Edward said. She said the kidnappers were interrogating the hostages about whether the women were members of local militias.
'They are facing death'
Edward said he feared the hostages would face the same fate as Assyrians targeted in Iraq and the more than 20 members of Egypt's Coptic Christian minority slaughtered by ISIS in Libya last month.
"Maybe they are facing the same destiny. That's why we call on all over the world, like the U.S, Europe, coalition forces -- protect Assyrians, save Assyrians in Syria," he said.
"They are facing death, people are unarmed, they are peaceful. And they need help, they are just left alone -- no one's protecting them."
Edward said some 35 Assyrian villages and towns had now been taken over by ISIS, forcing thousands of families to flee. Some 600 of these families have taken refuge in St. Mary's Cathedral in al-Hasakah, Syria.
After years caught up in the middle of a civil war, many of these Assyrians lack food, water, blankets and other basics.
The Assyrians are a proud group that's 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."
Kurdish fighters, coalition airstrikes going after ISIS
ISIS has proven, time and again, its willingness to ruthlessly go after minority groups who don't subscribe to its extreme, twisted 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.
Thus, while kidnapping women and children is unacceptable by any measure, ISIS going after the Assyrian community in al-Hasakah Province may have an element of military, as well as religious, motivation.
But ISIS has faced some opposition.
According to the Syrian Observatory for Human Rights, members of the Kurdish YPG -- or People's Protection Units -- have taken control of 70 villages in the Syrian province.
They're on the cusp of taking some inhabited by Assyrians and were clashing Wednesday with ISIS forces around Tal Tamer, the London-based monitoring group reported.
Embattled civilians are also getting help from the the al-Sanadid Army and other fighters, as well as from U.S.-led coalition airstrikes. The Observatory claims that 132 ISIS militants have been killed since Saturday.
Progress reported in Iraq's Anbar Province
Of course, this isn't the only place where ISIS has been blamed for atrocities or has encountered pushback.
The militant group also holds vast swaths of territory in Iraq, including its second-largest city of Mosul.
Yet Iraq's once beleaguered military has made progress of late, liberating most areas of the long besieged town of al-Baghdadi, according to the Iraqi Defense Ministry.
Videos released by the ministry Tuesday showed convoys moving toward al-Baghdadi and patrolling parts of the city as well as lawmakers visiting the nearby Ayn al-Asad air base, where some 400 U.S. military personnel are stationed to train Iraqi pilots in the fight against ISIS.
When contacted by CNN, an Anbar official, Faleh al-Assawi, said Iraqi security forces aided by U.S.-led coalition airstrikes had secured about 80% of al-Baghdadi. The Defense Ministry said on its website that it is working to secure the rest of the town.
Elsewhere in Anbar Province, video posted online shows Iraqi security forces' discovery of tunnels presumably dug by ISIS to give them an underground route to reach a central government compound in Ramadi, about 70 miles (110 kilometers) west of Baghdad.
This can all be seen as progress. Even then, they're only small steps toward eradicating a threat that, in many ways, remains as savage and dangerous as ever.