Belgian authorities and Islamic groups have become increasingly concerned about a growing number of young men leaving to fight in Syria against the regime of President Bashar al-Assad, some of whom are suspected of war crimes.
Tarik Taketloune, 20, a car mechanic from Vilvorde, a suburb of Brussels, was shot in the head and killed on Friday as he slept in a house near Aleppo, the Flemish newspaper Het Nieuwsbald said, quoting his father.
The house was occupied by members of Sharia4Belgium, an Islamist group which has sent several young men to fight in Syria. Sharia4Belgium has formed its own brigade which is under the command of the Al-Qaeda linked Al-Nusra front.
Taketloune's older brother Elias, 23, was arrested on his return to Belgium on June 3 and has been detained on suspicion of terrorist activities, their father said.
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Taketloune is the third Belgian to be killed while fighting in Syria, while there are unconfirmed reports of a fourth death, Belgian newspapers said.
He may also be among a group of men speaking in Flemish and French who feature in a recent video of a man being decapitated with a knife, the newspapers said.
His younger brother Hakim Elouassaki, 21, meanwhile told his girlfriend that he had shot a prisoner in cold blood, the reports said.
Hakim was seriously wounded in March and returned to Belgium, where he was arrested in April during a series of raids by Belgian authorities aimed at cracking down on fighters travelling to Syria.