With the fall of the last bastion of the self-proclaimed Islamic State (IS, formerly ISIS/ISIL) near the Syrian town of Baghouz, the United States has demanded European countries take back and put on trial hundreds of foreign fighters captured by the US-led coalition. The father of two young German men who travelled to Syria to join IS in 2014 opened up about his years-long quest to reunite with his sons, in an interview at his home in the town of Zierenberg, near Kassel, on Monday. He called on the German government to allow for their return.

The last time real-estate manager Joachim Gerhard saw his two sons, Fabian and Manuel (born in 1992 and 1996), was shortly before they secretly left for Syria at the end of October 2014. The two brothers had converted to Islam in April and May 2014.

More than four years later, at the end of February, Gerhard received a phone call from an unknown number saying that the eldest son, Fabian, was alive and a prisoner of the Free Syrian Army. "No one was asking for money. It was merely information that three days earlier Fabian had been imprisoned together with 20 more people," Gerhard said before adding that the detention centre was supposedly located in the area of al-Qamishli, near the Syria-Turkey border.

In March 2015, just a few weeks after Gerhard had travelled to Turkey in the hope of meeting up with his sons across the border in Syria, he received a message announcing they had died in Kobane. Fabian and Manuel have never been declared officially dead by any official German authority, according to their father who also refused to believe they are gone.

Gerhard's living room is covered with photos of his two sons, some portraying them in Syria: one depicts Fabian, the oldest, standing with black headgear against the backdrop of a lake. His left index finger points towards heaven, a symbol of IS militants, while he wears a rifle around his upper body.

More than 4,000 militants have surrendered to the SDF over the last month according to Mustafa Bali, spokesperson for the SDF Media Office.

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