The group said it learned of the killings from local sources in the city, which has been under ISIS control since May. The London-based Syrian Observatory for Human Rights relies on activists inside Syria to document human rights violations in the country .
Not much else is known about the executions, including the names of the men or what they were accused of.
But ISIS has killed for the slightest of offenses and used brutal ways to do so.
In another recent execution, ISIS ran over a member of the Syrian military with a tank "because he ran over dead bodies of the Islamic State soldiers by tank."
Some sentenced to death have been forced to dig their graves with their own hands before being put to death.
ISIS, also known as the Islamic State in Iraq and Syria, has taken over large swaths of both nations in an effort to create a caliphate. Where the group holds sway, it has imposed harsh Islamic Sharia law.
ISIS jihadists seized control of Palmyra, a UNESCO World Heritage Site, and the surrounding area from Syrian government forces on May 20. Since then, the Islamic extremists have beheaded the antiquities expert who looked after the ruins and set about demolishing their architectural riches.
In August, they leveled two temples of immense cultural significance -- the Temple of Bel and the Temple of Baalshamin -- prompting outrage and condemnation from around the world.
The Arch of Triumph, consisting of one large arch flanked by two smaller ones, opened onto Palmyra's elegant Colonnade. The top of the arch was decorated with "beautiful geometrical and plant ornaments," the Syrian antiquities directorate said.
The militants, however, are unapologetic over their step-by-step trashing of one of the world's most important ancient sites, publishing photos of some of the destruction
Palmyra, northeast of Damascus, is known as the "bride of the desert" for its magnificent collection of structures along a historical trade route that once linked Persia, India and China with the Roman Empire.