Isis has quietly executed hundreds of prisoners in Syria over the last two months, as it makes a desperate last stand to hold on to its remaining territory.

The extremist group has been cornered in a small pocket of territory on the eastern banks of the Euphrates river in eastern Syria, and US-backed forces recently sent the group fleeing from the last major town under its control.

But with nowhere left to run, the group has begun massacring prisoners it has been holding, and has killed some 700 in the past two months, according to the Syrian Observatory for Human Rights.

Despite having a history of using executions to gain notoriety and attention, the Observatory said these latest killings were carried out in secret and away from the cameras, and the bodies buried in mass graves.

The UK-based monitoring group said the prisoners were among 1,350 civilians and fighters that Isis had been holding, and that a number of Isis members who had tried to escape were among those killed.

It added that the group is still thought to be holding hundreds of prisoners, citing sources within Isis and residents who had left the area it still controls.

Isis has been putting up fierce resistance in its last redoubt – a string of villages in Deir Ezzor near the town of Hajin. After three months of intense fighting, the Kurdish-led Syrian Democratic Forces (SDF) announced last week that it had taken most of the town.

The capture of Hajin would represent a milestone for the SDF, which for the last four years has been the west’s main ally in the fight against Isis in Syria. With the backing of the US and UK, it has forced Isis from swathes of the country’s north and east to this small pocket.

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But the alliance was dealt a heavy blow on Wednesday by the decision of US president Donald Trump to withdraw all US forces from the country.

Top US officials, and the SDF, have warned that Isis will pose a threat even after it loses the last of its territory.

“The White House decision to withdraw from the north and east Syria would negatively impact the campaign to fight terrorism and it would give terrorism and its supporters, military field and political clout to revive itself again,” the SDF said in a statement on Thursday.