The Islamic State group has claimed to have beheaded a Lebanese soldier it captured in a raid on the Lebanese border town of Arsal last month, the second such murder in just over a week.
Images posted by the group's supporters on social media on Saturday appeared to show Abbas Medlej killed by masked men.
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If confirmed, Medlej would be the the second of 19 soldiers missing since the attack to have been executed.
Fighters from the IS group had previously killed Ali al-Sayyed on August 30.
A caption posted with the images identified Medlej as a Shia. Sayyed, who was buried on Wednesday, was a Sunni.
A source in the Lebanese army said it was investigating but could not confirm that Medlej had been killed. However, Medlej's maternal uncle, Abu Ali Noun, was reported by the AP news agency as saying the photographs appeared to be of his nephew.
Relatives of the troops demonstrated on Saturday to demand that the Lebanese government does more to secure their release.
A previously released video showing the execution of Sayyed included nine other soldiers begging for their lives. The fighters want the release of fellow fighters held at a Lebanese jail.
Some of the soldiers are being held by the Nusra Front, an al-Qaeda affiliate that also took part in the Arsal attack.
A number of Lebanese policemen are also being held by the fighters.
The incursion into Arsal marked the most serious spillover to date of the Syrian civil war into Lebanon and triggered five days of battles with the Lebanese army.
The IS group already holds large parts of Syria and Iraq.
In the past two weeks, the group released two videos showing the beheadings of American journalist James Foley and American-Israeli journalist Steven Sotloff, saying it was in retaliation for US air raids against the group in Iraq.