IS fighters seized the village of Al-Jaffrah, adjacent to the base, leaving it surrounded except for a narrow corridor to the west, the Syrian Observatory for Human Rights said.
IS forces control most of Deir Ezzor province, but half of its capital remains in government hands, along with the military airport.
The oil-rich province lies between IS-controlled Raqa province and the Iraqi border, and is a key prize for the jihadist group, which declared an Islamic "caliphate" straddling the two countries in June.
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IS launched a renewed assault on the military airport earlier this week.
The fighting has killed at least 30 government troops and militia, as well as 27 IS fighters, over the past 48 hours, the Observatory said.
Two of those killed on the government side were captured and beheaded, the Britain-based group said.
Further east, at least 15 jihadists were killed in a US-led coalition air strike on an IS convoy near the Albu Kamal crossing on the border with Iraq, it added.