BEIRUT: The Islamic State militants have abducted some 900 Kurds in the northern Syrian province of Aleppo over the past three weeks amid fierce fighting for control of a nearby IS stronghold, a Kurdish official and Syrian activists said Friday.

The abductions began shortly after the predominantly Kurdish and US-backed Syria Democratic Forces began an offensive on May 31 to capture the IS stronghold of Manbij from the extremists.

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

The Britain-based Syrian Observatory for Human Rights said the 900 civilians were detained near the IS-held town of Al-Bab. SDF spokesman Sherfan Darwish told The Associated Press that the seizure of over 900 is in retaliation for the SDF offensive on the town of Manbij.

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

The Observatory and Darwish said some of those taken captive have been forced to dig trenches in IS-held areas while others are imprisoned in IS-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 IS positions.

The Observatory 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.

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

Opposition activists reported clashes inside Manbij on Friday and airstrikes by the U.S.-led coalition.

Since the offensive to take Manbij began last month, SDF fighters have captured dozens of villages and farms near the town, which is now surrounded from all side as SDF forces slowly push into Manbij.

The town lies along the only IS supply line between the Turkish border and the IS extremist group's self-styled capital, Raqqa. If Manbij is captured, it will be the biggest strategic defeat for IS in Syria since July 2015, when it lost the border town of Tal Abyad.

The battle of Manbij has so far claimed the lives of 81 SDF fighters and 463 extremists, according to the Observatory.