The Lebanese army is investigating pictures and video footage of the alleged beheading of a Lebanese soldier by Islamic State jihadists, a military spokesman told AFP on Saturday.
IS supporters released a video showing the decapitation of a blindfolded man in military fatigues, and presented as one of 19 Lebanese soldiers abducted by jihadists in eastern Lebanon on August 2.
"We are investigating," a military spokesman said, without saying whether the video was thought to be authentic.
The soldier victim, named by the IS supporters as Ali al-Sayyed, is seen sitting on the ground with three armed and masked men around him.
He becomes visibly scared when one of the three announces that he is to be killed, and is then beheaded to cries of "Islamic State!"
One gunman charges that his death is in reprisal for the Lebanese army acting "under the orders" of Lebanon's Shiite Hezbollah movement, which is fighting alongside Syrian regime forces against rebels and jihadists.
"First decapitation of a Lebanese soldier by the lions of the Islamic State," IS backers wrote on Twitter.
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Three armed groups, including IS and Al-Nusra Front, the Al-Qaeda branch in Syria, have been holding 19 Lebanese soldiers and 15 police since the beginning of the month.
The men were abducted during clashes between the security forces and jihadists in Lebanon's Arsal region near the border with Syria.
Twenty soldiers and dozens of jihadists and 16 civilians were killed in the fighting.
The jihadists now want to exchange their hostages for Islamist prisoners held in Lebanon.
Another video circulating on Saturday on social networks shows nine captured Lebanese soldiers urging their families to put pressure on the Beirut government for their release, saying they will be beheaded in three days otherwise.
Relatives of the soldiers briefly sealed off roads in the north and east, demanding that the government negotiate with the kidnappers.
The war in Syria has regularly spilled over into Lebanon, and raised tensions between Sunnis who back the uprising there and Shiites who mostly side with President Bashar al-Assad.
On August 23, Al-Nusra Front posted a video showing kidnapped policemen calling on Hezbollah to withdraw its fighters from Syria.
IS jihadists, against whom the United States is seeking to build a global coalition, have carried out a wave atrocities in areas it controls in Syria and Iraq.
They do not hesitate to record on video and post on the Internet footage of captives being decapitated, including American journalist James Foley.