Isis fighters have kidnapped dozens of women and children in the eastern Syrian province of Deir ez-Zor, where the group is making a desperate last stand.

The captives were among some 250 civilians taken from the al-Bahra displacement camp, near the town of Hajin, after Isis fighters stormed the area on Friday and forced Kurdish fighters to flee, according to a war monitor.

The string of towns and villages on the eastern banks of the Euphrates river comprise the last populated areas under the control of Isis, in either Iraq or Syria, and are thought to contain some 15,000 people.

The area has been under attack by the US-led coalition and Syrian Democratic Forces (SDF) for the last month, but the offensive has been met with heavy resistance from around 5,000 hardened Isis fighters with nowhere else to run.

The tented camp in al-Bahra is in an area controlled by the SDF, and housed hundreds of civilians who had fled Isis-controlled territory, including families of Isis defectors and relatives of fighters who had been killed.

“The camp is not very far from the Isis territory, just a few hundreds metres from Hajin,” said Rami Abdulrahman, director of the Syrian Observatory for Human Rights, a UK-based monitoring group.

Since the offensive began on 11 September, around 7,000 civilians from Isis-controlled territory have fled to SDF areas, according to the United Nations – most from Hajin and Al Baghouz.

The observatory, which has a network of contacts across Syria, said the attack on the camp happened quickly and appeared to have been planned.

“They took control of the camp for a few hours and then the SDF took back control. Isis attacked and killed many SDF fighters. They took most of the people in the camp with them,” he told The Independent.

Mr Abdulrahman said they were in serious danger as Isis have been known to kill defectors and their relatives.

The remaining Isis fighters in the Euphrates valley are hardened combatants and have shown every indication of fighting until the end Colonel Sean Ryan, spokesman for the US-led coalition

The mass abduction by Isis is not the first of its kind this year. From its peak in 2014, when it controlled major cities in Iraq and Syria, the group has lost nearly all of its territory. But in retreat it has resorted to grabbing large numbers of civilians to give it bargaining power over advancing enemies.

More than 30 women and children were kidnapped by Isis fighters from the Druze town of Sweida in southern Syria during a devastating attack in July – most of whom are still held. Negotiations for their release have been ongoing, but the group has executed a number of the captives.

The tactic reflects a transformation over the past year, as Isis reverts from a proto-state into the underground insurgent organisation it was before 2014.

Although the group has suffered heavy defeats, a UN Security Council report released in August of this year estimated that the extremist group has an estimated 20,000 to 30,000 fighters across Iraq and Syria. Most of that number are not actively fighting but are “hiding out in sympathetic communities and urban areas” and could pose a threat again in the future.

The fighters in Hajin are different, however. An estimated 5,000 Isis fighters are bunkered down in the area, with nowhere else to run. The US military believes many top Isis leaders have fled there.

The group’s founder, Abu Bakr al-Baghdadi, was also rumoured to be in the area at one point. Operations in the area are somewhat complicated by the fact that Syrian government forces – who oppose US presence in the country – lie just across the Euphrates.

Russia completes delivery of S-300 air defence systems to Syria

The SDF, backed heavily by US forces, have captured three of the 10 towns currently held by Isis over the past month, according to Colonel Sean Ryan, a spokesman for the US-led coalition.

“The remaining Isis fighters in the Euphrates valley are hardened combatants and have shown every indication of fighting until the end,” he told The Independent. “The SDF have taken casualties but have inflicted great damage to command nodes, Isis vehicles, destroyed fighting positions, and of course have killed many Isis fighters.”

As the fighting continues, the threat to those still caught in Isis areas remains high. The UN's Office for Humanitarian Affairs said earlier this month that “scores of civilians have reportedly been killed and injured due to airstrikes or having been caught in the cross-fire” around Hajin.

“The ability of civilians to move freely remains unclear given ongoing high-intensity hostilities as well as persistent reports of restrictions imposed by parties to the conflict that prevent civilians from reaching safety,” it added.