Three Lebanese soldiers have been captured by gunmen near the border with Syria, sources have told Al Jazeera, days after the Nusra Front, a Syrian branch of al-Qaeda, 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 news came as Qatar announced that it would no longer continue its mediation efforts to release the 30 police and soldiers abducted by the Nusra group last August in Arsal. Four of the hostages had since been executed, and seven were released.

The Nusra Front had announced Qatar would no longer be welcome to the negotiations after the arrest of the wife and family of the armed group's commander, Anas Sharkas, in Lebanon.

The civil war in Syria has inflamed tensions in Lebanon's 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).

Al Jazeera's Jane Ferguson, reporting from the Lebanese capital Beirut, said that latest abductions had also caused tensions within the country's ruling coalition, with two dominant factions supporting different interests in Syria.

"It is an extremely tense time for this fragile coalition government, which is less than a year old," our correspondent said.

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 capturing 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.