ISIS has executed nearly 700 prisoners in nearly two months in eastern Syria, a war monitor said on Wednesday, as clashes between US-backed forces and militants continued in areas near the Iraqi border.

The UK-based Syrian Observatory for Human Rights says the prisoners were among 1,350 civilians and fighters ISIS had been holding in a shrinking strip of land east of the Euphrates River around the town of Hajin, which US-backed forces entered this month.

Wednesday's report comes nearly one week after seven mass graves containing hundreds of unidentified bodies were unearthed near the former ISIS stronghold of Albu Kamal in eastern Syria.

ISIS has been held responsible for multiple atrocities during its reign of terror, including mass executions and decapitations.

In recent months, both Syria and Iraq have discovered a number of mass graves in areas previously occupied by militants.

ISIS proclaimed a “caliphate” over swathes of the two countries in 2014, but has since lost almost all the territory it once held. However, it continues to command a pocket around Hajin, which is believed to be home to some high-profile ISIS militants, who fled former ISIS territories.

The Syrian Democratic Forces (SDF), backed by warplanes of the US-led coalition, have been battling ISIS in the area since September.

Battles between the SDF and ISIS erupted on the eastern flanks of Hajin on Wednesday, the Observatory reported, adding that coalition warplanes struck militant targets in the area.

The monitoring group said airstrikes have intensified since Tuesday.

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After months of battles, the SDF captured Hajin last week and pushed militants to the outskirts of the town, where clashes are now taking place.

At least 5,000 ISIS fighters remain holed up in the enclave, including many foreigners.

The Observatory estimates that around 950 ISIS militants and 545 SDF fighters have been killed in battles since September.

Meanwhile, more than 2400 civillians have fled towards SDF-held areas from ISIS territory in east Syria in less than a month, the Observatory says.

In recent weeks, the SDF had to draft in battle-hardened fighters from other fronts after devastating counter attacks by the extremists who used the cover of sandstorms to launch suicide bombings.