Islamic State militants have abducted 900 Kurdish civilians in Aleppo in retaliation for an offensive on a nearby stronghold.

The jihadis are reportedly forcing their prisoners to dig bunkers and trenches and are sending boys as young as 12 to fight on the front lines wearing ISIS uniforms.

At least 26 of those abducted have been killed for refusing to follow orders, reports reveal.

The abductions come amid fierce fighting for control of Manbij - a key ISIS stronghold in Syria.

The abductions come amid fierce fighting for control of Manbij - a key ISIS stronghold in Syria

Islamic State militants have abducted 900 Kurdish civilians in Aleppo in retaliation for an offensive on a nearby stronghold (file image)

Extremists there are being routed from the town center by the largely Kurdish and U.S.-backed Syria Democratic Forces.

IS militants began abducting Kurdish civilians in retaliation the offensive on Manbij three weeks ago, said SDF spokesman Sherfan Darwish.

'Whenever Daesh is defeated, they retaliate against civilians,' Darwish said, using an Arabic acronym for ISIS, adding that there are whole families among those taken.

The abductions have been taking place mostly in areas under IS control, from the western Manbij countryside to the towns of al-Bab and al-Rai.

The reports about the killing of captives could not be independently confirmed and the Islamic State group made no immediate claim over the abductions or the killings.

Many of the civilians are being held in an IS prison in Qabasin and those forced into labor are digging fortifications underneath homes in al-Bab, said Kurdish media activist Rezan Hiddo.

'They (IS) are digging a city underneath the town to protect themselves from air strikes,' Hiddo added.

The jihadis are reportedly forcing their prisoners to build shelters and trenches while all males older than 12 are given ISIS uniforms and sent to front lines (file image)

The IS has not engaged in any negotiations for the release of the Kurdish civilians, nor asked for any ransom, Hiddo said, speaking from the nearby Kurdish stronghold of Afrin.

On Friday, the mostly Kurdish Syrian Democratic Forces battled IS militants inside Manbij having encircled the stronghold in a weeks-long offensive.

The attacked was backed by U.S.-coalition air strikes which reportedly hit targets in the town.

The SDF has lost 89 fighters since launching its campaign for Manbij on May 31, but 463 IS militants have been killed.

The push by the SDF into Manbij has been slow as the U.S.-backed fighters first focused on capturing dozens of villages and farms near the town in the past weeks.

The town lies along the only ISIS supply line between the Syrian-Turkish border to the north and ISIS's capital Raqqa, to the southeast.

At least 26 of those abducted have been killed for refusing to follow ISIS orders, reports reveal (file image)

If Manbij is captured, it will be the biggest strategic defeat for IS in Syria since July 2015, when the extremist group lost the border town of Tal Abyad.

The U.S. has embedded 300 special forces operators with the SDF, which the White House says are advisers, and French special forces operators are also embedded with the group.

The extremist group has a history of mass kidnappings in areas they control in Syria and Iraq and has mostly targeted Christians and Kurds in the past.

The Britain-based Syrian Observatory for Human said that said IS fighters stormed homes in several villages they control near al-Bab, including Arab, Qabaseen and Nairabiyeh, and took with them mostly men.

In 2014, IS abducted nearly 200 Kurdish students near Manbij as they were en route from the Kurdish town of Kobani near the Syrian-Turkish border to the northern Syrian city of Aleppo, the provincial capital, to take their exams. Most were later released.