BEIRUT, Lebanon — Members of the Islamic State in Iraq and Syria have beheaded a Lebanese soldier who was one of 19 captured when militants seized a Lebanese border town for a few days this month. A video of the beheading was posted on social media sites on Saturday.

The soldier, recognizable as Ali al-Sayyed, a Sunni Muslim from northern Lebanon, was shown blindfolded with his hands tied behind his back, writhing and kicking the dusty ground as a militant announced that he would be killed. Another militant then beheaded him.

The Lebanese Army declined to comment, but security officials and members of ISIS confirmed the beheading.

Hours later, the group posted a second video showing nine other soldiers begging for their lives — urging their families to stage street protests to demand that Lebanese officials release Islamist prisoners, so the soldiers would not be killed.