Syrian troops seized a suburb of the city of Deir al-Zor in eastern Syria on Sunday, tightening the noose around Daesh militants, a military source said.
Oil-rich Deir al-Zor province, which borders Iraq, is Daesh's last major foothold in Syria.
The Syrian army pushed into Deir al-Zor city this month, with Russian air power and Iran-backed militias, breaking an Daesh siege of an enclave that had lasted three years.
Russia's RIA news agency cited an unnamed source as saying that the Syrian army cut Daesh's main supply line in the city on Sunday after taking control of the al-Jafra district.
The Syrian military source said the army and allied forces captured al-Jafra on the western bank of the Euphrates. Daesh militants could only escape across the river.
"They have no outlet except crossing the Euphrates towards the eastern bank and fleeing towards the desert, or (the towns of) al-Bukamal and al-Mayadin," the source told Reuters.
The British-based Syrian Observatory for Human Rights said the army and its allies took al-Jafra and other villages near the city's air base overnight.
Daesh militants still hold nearly a third of the city, the war monitoring group said. Russian jets pounded movements across the river as Daesh fighters tried to escape in ferries, it added.
Many civilians, including families of the militants, had also tried to flee through the river in recent days, the monitor said.
Separate air strikes by Russia and by the U.S.-led coalition had killed more than 30 people across Deir al-Zor province over the past day, it said.
Daesh has fallen back on towns downstream of Deir al-Zor and controls much of the desert region around the city, near the border with Iraq. Its fighters have also come under attack there from a separate U.S.-backed offensive.
An alliance of Kurdish and Arab militias is battling Daesh in the northern parts of Deir al-Zor province, with jets and special forces from the U.S.-led coalition.
The Syrian Democratic Forces alliance said it had taken 14 villages and farms, two large towns, and some factories on the eastern bank of the Euphrates since launching its assault last week.
The two offensives have advanced from opposite sides of the river, which bisects Deir al-Zor province and has often served as a dividing line between Russian and U.S. battles against Daesh in Syria.