Story highlights Hundreds of combatants and civilians have been killed over the past week

The regime of President Bashar al-Assad is battling to retain a foothold in the area

Russian aircraft have been dropping supplies to beleaguered army units

(CNN) Fierce battles continue around the strategic city of Deir Ezzor in northern Syria between regime forces and the Islamic State in Iraq and Syria (ISIS), according to human rights groups.

Hundreds of combatants and civilians have been killed over the past week, and Russian aircraft have been dropping supplies to beleaguered army units.

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The regime of President Bashar al-Assad is battling to retain a foothold in the area. It still controls the military airport to the south, but ISIS claims to have overrun several regime-held districts at the beginning of the week, taking advantage of a sandstorm that grounded military aircraft. But in the last few days, Russian and possibly Syrian warplanes have carried out airstrikes against ISIS areas, while the already desperate situation of civilians has worsened.

The Institute for the Study of War -- a Washington-based group that analyzes the conflict in Iraq and Syria -- said Russia had shifted the focus of its air campaign to Deir Ezzor in the face of the major ISIS assault on parts of the city still held by the regime. "The shift in Russian air operations serves to forestall the immediate defeat of regime forces in Deir Ezzor, one of the last remaining regime outposts in Eastern Syria," the Institute said.

Even so, the regime presence in Deir Ezzor looks increasingly precarious. Were ISIS to capture the entire city, it would give the group morale-boosting control of a second provincial capital in Syria, after Raqqa, and improve its supply lines across the border into Iraq.

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