Fierce battles continue around the strategic city of Deir ez-Zor in northern Syria between regime forces and the Islamic State in Iraq and Syria (ISIS), CNN reports citing 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.

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 ez-Zor 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, according to CNN.

Even so, the regime presence in Deir ez-Zor 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.

Much of the fighting has concentrated on the suburb of al Bghiliyyeh to the north of the city, an area held by army units and allied militia even after ISIS took control of most of Deir ez-Zor a year ago. The Syrian Observatory for Human Rights (SOHR), which monitors the conflict across Syria, reported that 200 soldiers had been killed in the latest surge of fighting, of whom 48 had been executed by ISIS, while 110 ISIS fighters had died, including 30 suicide bombers.

SOHR said that more than 40 civilians had been killed in aerial and artillery bombardments by regime forces and their allies as they tried to recover lost ground. And on Friday it reported fresh airstrikes against the nearby town of Tabia, which is controlled by ISIS, in which 30 civilians were killed, including 13 children.