The fighting came as regime forces battled IS in the northern province of Aleppo, repelling a jihadist assault and killing at least 16 fighters from the group.
The Syrian Observatory for Human Rights said IS had advanced into the northern tip of Deir Ezzor city, in eastern Syria, and captured the suburb of Al-Baghaliyeh.
The advance puts IS in control of around 60 percent of the city, with the regime holding the rest, according to the Britain-based monitor.
Syrian state news agency SANA said regime troops had repelled an IS attack on the area around Al-Baghaliyeh and inflicted “heavy losses” on the group.
Deir Ezzor is the capital of Deir Ezzor province, an oil-rich region that borders Iraq and is mostly held by IS.
The regime has clung onto portions of the provincial capital and the adjacent military airport despite repeated IS attacks.
Observatory chief Rami Abdel Rahman said heavy fighting was continuing on Saturday afternoon after the IS assault, which began with a suicide car bomb blast carried out by a member of the jihadist group.
Eight of the regime forces killed were shot dead by IS jihadists, the Observatory added.
The monitor said Russian warplanes were carrying out heavy air strikes in support of regime forces as they sought to repel the jihadists.
Elsewhere, regime troops were locked in fierce clashes with IS in Aleppo province, with at least 16 jihadists killed after a failed attack on a government position near the town of Al-Bab, the Observatory said.
State television also reported that regime forces had repelled an assault.
The Observatory said heavy fighting was ongoing throughout Saturday in the area, with Russian war planes carrying out strikes in the region between the regime-held Kweyris air base and Al-Bab.
– Seven battlefronts -The regime has advanced towards the town, an IS bastion, in recent days, and is now within 10 kilometres (six miles) of it, according to the Observatory.
That is the closest regime forces have come to Al-Bab since 2012.
The Britain-based monitor also said regime forces had taken a string of villages nearby.
Roughly 30 kilometers (25 miles) south of the Turkish border, Al-Bab fell into rebel hands in July 2012, and IS jihadists captured it in late 2013.
The fighting in Al-Bab is just one of up to seven fronts on which regime forces are seeking to advance in Aleppo province, capitalizing on a Russian air campaign that began on September 30
The various battles are intended in part to cut rebel supply lines into Aleppo city, the provincial capital and Syria’s second city.
The city itself is divided and regime forces are now hoping to effectively encircle the opposition-held east.
In addition to cutting rebel access to eastern Aleppo city, the regime is hoping to sever areas controlled by IS in the province from its territory in neighboring Raqa, Abdel Rahman said.