A suicide car bomber reportedly killed three people at a checkpoint manned by the Shia group Hezbollah in eastern Lebanon, the state-run news agency reported.

However, a Hezbollah-run TV station reported later on Saturday that the suicide bomber did not kill anyone at the checkpoint in the Bekaa Valley. There was no immediate way to reconcile the contradictory reports.

The bomber detonated his explosives-rigged vehicle at the checkpoint about 5km from the Syria border near the town of Khreibeh, Lebanon's National News Agency said.

It was not immediately clear if the reported casualties were bystanders or Hezbollah fighters.

Since July 2013, dozens of people have been killed in more than a dozen suicide car bombings that have rocked the country.

The attack on Saturday was likely linked to the conflict in Syria, as the spillover from the war raises sectarian tensions in the Mediterranean country, where Sunnis and Shia back opposing sides of the neighbouring conflict.

Sunni fighters, including the Islamic State in Iraq and the Levant (ISIL), have been trying to punish the Iranian-backed Hezbollah because its fighters are battling alongside forces loyal to Syrian President Bashar al-Assad.

They have claimed responsibility for previous bombings in Lebanon that have targeted Shia communities. But they have also increasingly targeted the Lebanese army itself, seeing it as aligned with Hezbollah fighters.

Tensions have ratcheted further in recent weeks, after fighters from Syria overran a Lebanese border town, Arsal, in early August, capturing and killing soldiers and police.

The fighters, including the al-Qaeda linked Nusra Front and ISIL, are holding about 20 Lebanese hostages.

At least three have been killed by their captors. Two have been beheaded by ISIL, sparking days of violence against Syrian refugees in Lebanon and tit-for-tat kidnappings between Sunnis and Shias.

Lebanese soldier killed

News of the killing of a third soldier surfaced on Saturday, when the father of one of the men held captive by the Nusra Front said he believed that his son had been killed by the fighters.

Maarouf Hamieh said his son Mohammad was shot to death after a video appeared onlilne showing the soldier repeating the demands of his captors before being shot.

"It's confirmed," the father said grimly in an interview with The Associated Press.

In the video, a man identified as Mohammed Hamieh is shown kneeling in what appears to be an orchard, dizzily swaying as he tries to breathe, pleading that Hezbollah fighters must leave Syria or otherwise his captive comrades would be killed.

"I am paying the price now, and all of my friends will die, all of them," he said, his voice occasionally breaking into sobs.

As he spoke, a masked man sat silently behind him, and seconds later shot the Lebanese soldier in the back of his head. A captive Lebanese policeman then was shown, his hands handcuffed, pleading with the Lebanese government to speed up negotiations or he would be killed next.

Twitter messages by a Nusra Front account said that Hamieh was "sacrificed" and warned that the policeman would be next.

Another tweet on a Nusra Front account said the al-Qaeda linked rebel group was open to negotiations to secure the release of the Lebanese captives, but that so far all negotiations had failed.

At least seven of the captured Lebanese soldiers and policemen are being held by the Nusra Front, while the others are being detained by ISIL.

Following the video's release, angry Shia residents near Hamieh's hometown kidnapped Sunnis from a neighbouring village, even as his family pleaded for calm.