Zeinab Noun called her son, Abbas Medlej, a "martyr" after Islamic State supporters today posted images appearing to show a captured Lebanese soldier before and after he was beheaded.
Medlej's maternal uncle, Abu Ali Noun, also said the photographs appeared to be of his nephew.
A spokesman for Lebanon's military said it was still investigating the incident.
Militants from the Islamic State and other groups in Syria are holding around 35 Lebanese soldiers and policemen captive.