Several pro-jihadist Twitter accounts on Saturday published gruesome pictures apparently showing Islamic State militants beheading another Lebanese army soldier, around ten days after the same group executed captive army sergeant Ali al-Sayyed.

“We executed another army soldier, Abbas Medlej, after he tried to escape,” Turkey's Anatolia news agency quoted an unnamed IS leader as saying.

The so-called Qalamun branch of the Islamic State also published a statement confirming the execution of Medlej.

“Today, Lebanese soldier Abbas Medlej tried to escape from his prison in a malicious manner,” the group said in a statement circulated on social media websites.

“After he tried to open fire at ... the Islamic State's soldiers, we managed with God's help to control the situation and beheading was the fate of this Rafidi (derogatory term for Shiites),” the group added.

Meanwhile, pictures posted on social media showed a masked gunman cutting off Medlej's head and a number of jihadists surrounding the decapitated body.

In one of the pictures, a hooded man wearing black appears holding the severed head of a bearded young man over his body.

The corpse is lying in a pool of blood and, in the background, a man is holding up the black IS flag.

A military source said the Lebanese army is investigating the photographs.

"The army has received the images, but we cannot confirm or deny whether they are authentic," the source told Agence France-Presse, speaking on condition of anonymity.

The pictures emerged four days after DNA testing confirmed that the body of a man whom jihadists had said they beheaded was that of Lebanese sergeant Ali al-Sayyed. The images of al-Sayyed's execution were posted on the Internet on August 28.

He disappeared from Arsal around the time Syria-based militants from the IS and al-Nusra Front overran Arsal in early August during deadly clashes with the Lebanese army.

The fighting ended a few days later but the militants kidnapped around 36 troops and policemen and took them to an area between Arsal's outskirts and Syria's Qalamun.

The militants are reportedly seeking to negotiate the release of Islamist inmates from Lebanon's prisons in return for the abducted troops.

Medlej's execution comes despite the fact that Qatari mediators had held a meeting with IS militants earlier in the day and received a list containing their demands.

Lebanese officials have rejected holding talks on a possible prisoner swap, but have said Qatar is mediating negotiations on the issue.

As news broke that Medlej was executed, angry protesters took to the streets and blocked roads in Beirut's southern suburbs, Bekaa's Ablah, al-Labweh, al-Ain, and the northern town of Halba.

Earlier on Saturday, the body of a Lebanese civilian kidnapped and killed by the IS was handed over to his family in Arsal.

Kayed Ghadada was abducted last month after the Arsal battle, which killed 20 soldiers, dozens of jihadists and 16 civilians.

The fighting in Arsal was the most serious border incident since the Syrian conflict erupted in March 2011.

Y.R.