Islamic State (ISIS) militants abducted about 900 Kurdish civilians in the northern Syrian province of Aleppo over the past three weeks, forcing the captives to build fortifications for the extremists in retaliation for a Kurdish-led assault on a nearby ISIS stronghold, activists said Friday.

Reports also emerged Friday that at least 26 of those abducted have been killed for refusing to follow ISIS orders.

The abductions come amid fierce fighting for control of Manbij -- a key ISIS stronghold in this Syrian province -- where the extremists are being routed from the town center by the predominantly Kurdish and U.S.-backed Syria Democratic Forces.

Some of the abducted Kurds have been pressganged into digging trenches and shelters for the IS, according to Kurdish media activist Rezan Hiddo, while others have been forced to wear IS uniforms and ordered to fight at the fronts.

On Friday, the mostly Kurdish SDF battled IS militants inside Manbij after having encircling the stronghold in a weeks-long offensive that has been backed by U.S.-coalition air strikes.

The Britain-based Syrian Observatory for Human Rights monitoring group reported that coalition jets struck targets around the town. The SDF has lost 89 fighters since launching its campaign for Manbij on May 31, according to the Observatory. Also, 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 the ISIS extremist group's self-styled capital of Raqqa, which lies to the southeast, in the ISIS-held neighboring province of Raqqa.

If Manbij is captured, it will be the biggest strategic defeat for ISIS 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. The White House says they are advisers. French special forces operators are also embedded with the group.

SDF spokesman Sherfan Darwish told The Associated Press that the ISIS militants began abducting Kurdish civilians in retaliation the offensive on Manbij.

"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 ISIS control, from the western Manbij countryside to the towns of al-Bab and al-Rai, according to Hiddo. He said the ongoing campaign has prompted families to flee the towns in fear.

The Observatory and Darwish said some of those taken captive have been forced to dig trenches in ISIS-held areas while others are imprisoned in ISIS-run detention centers. Darwish said all captured males above the age of 12 were sent against their will to the front lines to help fortify ISIS positions.

The militants have killed 26 of the captives for resisting detention or refusing orders, according to Hiddo. His report 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 ISIS prison in Qabasin, Hiddo said. Those forced into labor are digging fortifications underneath homes in al-Bab.

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

ISIS 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.

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 Observatory also said that said ISIS fighters stormed homes in several villages they control near al-Bab, including Arab, Qabaseen and Nairabiyeh, and took with them mostly men.

In 2014, ISIS 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.

In February 2015, ISIS kidnapped more than 200 Christians from northeastern Syria. The Christians were released over a period of a year, after ISIS collected millions of dollars in ransom.