Syrian Observatory for Human Rights head Rami Abdel Rahman said clashes and at least one suicide explosion had rocked eastern neighbourhoods of Deir Ezzor controlled by regime forces.
The jihadists also clashed with government troops on the outskirts of Deir Ezzor's military airport, but Abdel Rahman said they had yet to make any advance on the ground.
IS already controls most of Deir Ezzor province and roughly half of its provincial capital, said local media activist Mohammed al-Khleif.
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"The most violent clashes between IS and the regime now are in the neighbourhoods of Al-Sanaa, Al-Rasafa, and Al-Omal in the east of the city," Khleif told AFP.
He said the areas were strategic because of their proximity to Deir Ezzor military airport and to the regime's "security quarter" in the city's west, besieged by IS for more than four months.
If IS succeeds, Deir Ezzor would be the second provincial capital to fall to the group, after it named the northern city of Raqa the capital of their "caliphate."
Abdel Rahman said Wednesday's clashes had killed one regime officer and three soldiers, but he could no details on IS casualties.
Deir Ezzor lies at the crossroads of several key highways in Syria, leading east to the Iraqi border and north to Hasakeh province, which borders Turkey and where IS has been locked in battle with Kurdish militia.