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 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.
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 Ezzor 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.
Inevitably, the Syrian Foreign Ministry has painted a very different picture, claiming ISIS had carried out a massacre in al Bghiliyyeh last Saturday, "slaughtering, and shooting innocent civilians, claiming the lives of more than 280 civilians, most of them women, children, and elderly people."
There is no way to independently reconcile the widely differing accounts of events in Deir Ezzour. But it's clear there has been heavy fighting and that ISIS has gained the upper hand, taking a number of districts as well as high ground around the city.
Videos and photographs published by the ISIS-affiliated Amaq news agency this week purport to show captured and dead regime soldiers, stocks of seized weapons, including a number of missiles, and ammunition.
Another Amaq video purports to show Russian planes dropping supplies to regime-held areas this week, but intelligence consultancy Flashpoint Partners said that given the fluid situation on the ground, "it is not clear whether the alleged Russian airdrops ended up in The Islamic State's hands."
The humanitarian situation in Deir Ezzor has worsened dramatically since ISIS besieged several neighborhoods a year ago. SOHR says ISIS has long prevented food and medical supplies reaching these areas and estimates that some 250,000 civilians in the area now lack food and other essentials. The latest fighting has sent thousands fleeing to surrounding areas.
ISIS appears to be trying to take advantage of its gains by coercing local tribes to take its side. SOHR said that according to its sources, ISIS had met with tribal leaders near the city this week to demand that "the sheikhs of the clans and their clans support the Islamic State in the fighting against the infidels." SOHR's sources said several agreements had been reached, without disclosing details.
Deir Ezzour is a critical junction for the group, with roads east and south toward Iraq and west to areas it controls in Homs province, including Palmyra. The city is also surrounded by some of ISIS' most valuable oilfields, which have been intensively targeted by both U.S. and Russian airpower in recent months.