A suicide car bomber exploded his vehicle at an army checkpoint in eastern Lebanon near the border with Syria on Saturday, killing at least three soldiers and wounding four, the army said.
A statement said the attack took place in the Arsal region "killing three soldiers and wounding four others".
The military said more troops fanned out across the region after the blast.
The security official told AFP the attack was at Aqabet al-Jurd and that at least nine soldiers were at the checkpoint.
The attack was claimed on Twitter by a shadowy group calling itself Liwa Ahrar al-Sunna in Baalbek -- Arabic for the Brigades of the free Sunni Muslims -- which also vowed more attacks.
"The next few days will see several jihadist and blessed attacks like this one. This is only the beginning," the group said, adding that the army would be among its targets.
It said Saturday's attack was to avenge the death of Sami al-Atrash, a suspect wanted in connection with car bombings targeting Lebanon's Shiite militant group Hezbollah.
Atrash was killed on Thursday in a shootout with the army.
The army had described Atrash as a "dangerous terrorist" who was tracked down to a hideout in Arsal, where he was killed.
After his death Liwa Ahrar al-Sunna pledged retaliatory strikes, saying "the gates of hell would open up" for the military and that those killing soldiers would go to heaven.
The group accuses the army of "targeting" Sunni Muslims and says it will "not tolerate" such action.
Attacks on the security forces have also taken place in the northern city of Tripoli, which has been rocked by violence linked to the Syrian conflict.
Two masked men killed a soldier there on Thursday.
The Arsal region has also seen a spillover of the conflict across the border as Syrian regime forces backed by fighters from Hezbollah press their campaign against rebels.
Saturday's attack came as Hezbollah chief Hassan Nasrallah gave a speech in which he said the group's involvement in the Syria conflict was aimed at protecting Lebanon from Muslim extremists.
Most Sunni Muslims in Lebanon back the rebels fighting to oust Syrian President Bashar al-Assad.
Sunni groups have targeted Hezbollah interests in Beirut and elsewhere in Lebanon with car bombings over the past few months.
Hezbollah claims that the car bombs were prepared in the Qalamun region just across the border before being driven via Arsal to their eventual targets.
Syrian troops backed by Hezbollah have gained ground in Qalamun, in mid-March seizing Yabrud, the last major rebel stronghold.
On Saturday, Syria's military said troops seized two villages from the rebels as they pressed their offensive with Hezbollah support.
A Syrian military source called the latest advance "a new step towards closing off the border with Lebanon".
Liwa Ahrar al-Sunna also claimed a March 16 car bombing in eastern Lebanon that killed two people.
The same attack was also claimed by Al-Nusra Front, Al-Qaeda's Syria affiliate, with both groups saying it was in revenge for the fall of Yabrud.
Nasrallah on Saturday said events in Syria prove that Hezbollah was right to help Assad's regime battle jihadist Sunnis dubbed "terrorists" by the Damascus government.
"Day after day, events prove that our choice was the right one and that if extremist terrorism had won in Syria we would have all been eliminated," he said.