In the two-day assault on the base in Raqa province, an IS bastion, the jihadists killed at least 85 soldiers, the Syrian Observatory for Human Rights said.
Hundreds of troops “withdrew on Friday to safe places — either to nearby villages whose residents oppose IS or to nearby Brigade 93 — but the fate of some 200 remains unknown,” Observatory head Rami Abdel Rahman said.
“Some of the executed troops were beheaded, and their bodies and severed heads put on display in Raqa city,” an IS stronghold, he told AFP.
Video shot by jihadists and distributed on YouTube showed IS fighters apparently inside Division 17 living quarters burning a portrait of Syrian President Bashar al-Assad.
The jihadists also posted photographs online of the bodies of decapitated soldiers strewn across the ground.
In one, six bloodied heads were lined up together on the ground, and in another three heads lay in a field.
Pictures of headless bodies, most wearing military uniform, were also distributed on the Internet.
Abdel Rahman said IS intended the display as “a message to the people of Raqa, to tell them it is strong, that it isn’t going anywhere, and to terrify” opponents.
Also in northern Syria, 30 troops and pro-regime paramilitaries were killed in an overnight ambush in Aleppo province, the Observatory said.
IS, which first emerged in Syria’s war in spring 2013, has since imposed its near-total control in Raqa province and Deir Ezzor on the Iraq border.
In June, the jihadist group proclaimed an Islamic “caliphate” straddling Syria and Iraq.
“There is a clear shift in the IS strategy. It has moved from consolidating its total control in areas under its grip. It is now spreading,” said Abdel Rahman.
“For IS, fighting the regime is not about bringing down Assad. It is about expanding its control,” he said.
IS was emboldened by a June offensive in Iraq when swathes of the north and west fell out of Baghdad’s control.
Syrian rebels say IS transported a large amount of heavy weapons captured from fleeing Iraqi troops into Syria.
On Friday, Brazilian Paulo Pinheiro, who heads the UN Commission of Inquiry on Syria, said IS “are good candidates for the list” of possible war crime indictees.
Syria’s Al-Qaeda affiliate the Al-Nusra Front meanwhile released a video of a young US suicide bomber from Florida who blew himself up at an army post in the northwest on May 25.
Moner Mohammad Abu Salha, alias Abu Hurayra al-Amriki, was believed to be the first American national to carry out such an attack in Syria’s more than three-year-old war.
The Observatory also reported six children and three women were among 15 civilians killed on Friday in rebel mortar fire on army-held areas of Aleppo city.
Regime air strikes, meanwhile, killed seven civilians, including three children, in opposition-held areas of Aleppo.
Once Syria’s commercial capital, the northern metropolis has been divided into regime and rebel-held areas since July 2012.
Elsewhere in Aleppo province, a child was killed when a regime helicopter was shot down over the army-controlled Palestinian refugee camp of Nairab, the Observatory said.