The Syrian Observatory for Human Rights reported
that the terror organization carried out the mass killing in the the al-Mayadin desert near the strategic city of Deir Ezzor on Wednesday morning, it said, adding that its activists were "able to monitor" the incident.
The London-based monitoring group called it "the largest execution operation carried out by the Islamic State organization in 2017."
The report said the people were between ages 18 and 25 and were "killed by sharp tools." It added that it is unknown whether the victims were Syrian government forces, allied militia or rebel factions.
The report came as ISIS killed at least 22 people in the Iraqi city of Tikrit, also on Wednesday.
ISIS gunmen indiscriminately opened fire on police and civilians in the central Iraqi city before they blew themselves up, police officials told CNN. At least 31 other people were wounded in the attack.
Several ISIS suicide bombers dressed in military uniforms attacked police checkpoints and police patrols in a busy commercial street in the city, police officials said.
ISIS claimed responsibility in a statement released on Twitter and tweeted by several ISIS supporters.
Tikrit, the birthplace of former Iraqi leader Saddam Hussein, was recaptured by Iraqi troops from ISIS in March 2015.
The jihadist group, which controlled swathes of Syria and Iraq since a blitzkrieg across the two countries in 2014, has steadily been losing ground thanks to concerted efforts by troops, and militia in both countries.
Nearly three years since the group's elusive leader Abu Bakr al-Baghdadi declared a self-styled Islamic State in Iraq and Syria, ISIS is reeling from losses across its so-called "caliphate."
Over the last six months, ISIS has seen its finances slashed, media operations crippled and several high-ranking leaders killed or captured.
It is fast losing its grip on Mosul, its biggest hub in Iraq, and its de-facto capital in Syria -- Raqqa -- is all but surrounded.
In Iraq, government troops, supported by Shia and Kurdish militia, have been making good progress in liberating Mosul from ISIS, which it has held since 2014.
And in Syria, the Syrian Democratic Forces -- an alliance of Kurds and Arab tribes -- are approaching the outskirts of Raqqa.