Three Lebanese soldiers have been captured by gunmen near the border with Syria, sources told Al Jazeera, days after the Syrian branch of al-Qaeda, the Nusra Front, executed a Lebanese policeman it was holding captive.

The three soldiers were reportedly taken on Monday morning in the town of Brital in the Bekaa Valley, which borders Syria, after the gunmen infiltrated an army position.

The civil war in Syria has inflamed tensions in the Bekaa Valley, where Sunni Muslims who support the uprising against Bashar al-Assad live close to Shia who back the Syrian president.

Violence has flared in the surrounding areas, where the Lebanese army has also battled Syria-based fighters, including the Nusra Front and the Islamic State of Iraq and the Levant (ISIL).

In August, fighters from the Nusra Front and ISIL stormed Arsal in the worst spillover of violence to date, killing at least 17 soldiers and captured more than 20 soldiers and policemen in a brief cross-border raid.

On Friday, the Nusra Front killed police officer Ali al-Bazzal and said it did so in response to the arrest of family members of its commanders.

Bazzal was the fourth hostage to be killed by his captors, two other hostages had been beheaded.

The plight of the remaining security officers has gripped Lebanon in the past weeks. Relatives of the detained are staging an ongoing sit-in and blocking roads in Beirut and elsewhere in Lebanon, to pressure the government to negotiate with the fighters.