The Syrian Observatory for Human Rights said the extremist group was engaged in heavy fighting with troops at Brigade 93 in Raqa, where IS already controls the regional capital and significant amounts of territory.
The monitor said at least 27 regime forces had been killed, along with 11 jihadists, including three who launched the assault by blowing themselves up at the entrance to the base and around it last night.
The Observatory said IS fighters "were in control of large parts of the base".
That assault left nearly all of Raqa province under IS control, except Brigade 93 and Tabaqa military airport, which were still held by the army.
The two-day assault on Division 17 saw more than 50 troops summarily executed, including some who were beheaded, with jihadists later putting their severed heads on display.
IS, which first emerged in Syria's war in spring 2013, has since imposed near-total control in Raqa province and Deir Ezzor on the Iraq border.
Some Western-backed rebels seeking President Bashar al-Assad's ouster initially welcomed IS as a potential ally.
But the extremist group's abuses and quest for domination has turned the opposition against it.
Syria's war has killed more than 170,000 people since March 2011.
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