Across the border the jihadists claimed to have abolished when they proclaimed their "caliphate" last year, thousands of Iraqi security forces and paramilitaries deployed across Anbar province.
Nearly a week after seizing strategic Palmyra, IS gathered 20 men they accused of fighting for the regime in the ruins of the theatre and shot them dead, Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP.
"IS gathered a lot of people there on purpose, to show their force on the ground," he said.
The jihadist group has damaged priceless historical sites across the region but mainly used its sledgehammers and dynamite on statues and places of worship it considers idolatrous.
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IS seized Palmyra on May 21, a move analysts warned positioned the group to launch more ambitious attacks on Damascus and third city Homs.
According to the Observatory, it has over the past week executed at least 217 people, including 67 civilians, in and around the city.
Their immediate goal was to cut off the jihadist group's supply lines, but some forces inched towards provincial capital Ramadi which IS captured on May 17.
The fall of the city, 100 kilometres (60 miles) west of Baghdad, was a huge blow to the government and its policy of building up a local Sunni force to expel IS from its bastions.
Nonetheless, 1,000 members of a newly formed Sunni unit graduated and received weapons at an event in Anbar's Habbaniyah base that had been delayed by the fall of Ramadi.
"Iraqi security and Hashed forces took control of both neighbourhoods. They also managed to enter the university but have yet to liberate it," he said.
Hashed al-Shaabi is an umbrella group for mostly Shiite militias and volunteers that the government called in after Ramadi fell to IS.