A suicide car bomber has exploded his vehicle at an army checkpoint in eastern Lebanon near the border with Syria, killing at least three soldiers and wounding four.
The attack took place on Saturday in the Arsal region, according to army officials.
They said more troops fanned out across the territory after the blast.
One of the officials told Agence France-Presse the strike occurred at Aqabet al-Jurd and that at least nine soldiers were at the checkpoint.
The attack has been 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 Shi'ite militant group Hezbollah.
Atrash was killed on Thursday in a shootout with the army after being tracked to a hideout in Arsal.
He had been described as a "dangerous terrorist".
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.
