message Wednesday threatening to kill them.
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.
Initial estimates put the number seized at between 70 and 100 people, among them women, children and the elderly.
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 Tuesday 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, an ISIS stronghold.
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.