Belgian authorities have become increasingly concerned about a growing number of young men leaving to fight in Syria against President Bashar al-Assad government.

Tarik Taketloune, 20, was shot in the head and killed on Friday in a house near Aleppo, the Flemish newspaper Het Nieuwsbald said.

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.

Taketloune is the third Belgian to be killed while cooperating with militant groups fighting in Syria, while there are unconfirmed reports of a fourth death, Belgian newspapers said.

Separately the leader of the Belgian terrorists in Syria, named as Elouassaki, said in a conversation recorded by police that he had slit the throat of a Syrian soldier, according to the reports.

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.

Europe has seen a rise in the number of its citizens entering Syria as ‘jihadists.’

On April 24, EU’s anti-terror chief Gilles de Kerchove told the British media that some 500 Europeans were in Syria to fight against the government of President Bashar al-Assad.

Kerchove said Britain, Ireland and France were among the EU countries to have the highest numbers of militants in Syria.