A suicide bomber killed himself and three soldiers when he detonated a car bomb at a Lebanese army checkpoint in the border town of Arsal, a security official said.
The Lebanese army said in a statement that four other soldiers were wounded in Saturday's attack on the checkpoint at Aqabet al-Jurd in the Arsal area.
The attack was claimed on Twitter by a group calling itself Liwa Ahrar al-Sunna in Baalbek, Arabic for the Brigades of the free Sunni Muslims.
It said the attack was to avenge the death of Sami al-Atrash, a suspect wanted in connection with car bombings targeting Lebanon's Shia armed group Hezbollah.
Atrash was killed on Thursday in a shoot-out 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.
Lebanon's army vowed to battle "terrorism" hours after the car bombing.
"The army knows that it is now more than ever targeted by terrorists who want to prevent the establishment of the authority of the state and its attempts to eliminate discord," the military said in a statement issued late on Saturday.
Woman and child killed
Later on Saturday a woman and a child were killed when soldiers opened fire on their car after it failed to stop at another checkpoint at the entrance to Arsal town, an army spokesman said.
The bodies were brought to a makeshift hospital in Arsal, Lebanon's national news agency reported, adding that the male driver of the car was in serious condition.
Arsal is home to thousands of Syrian refugees but also Syrian rebels and their Lebanese allies who have fled a Syrian army advance on the Syrian side of the border.
Lebanese Sunni fighters accuse the Lebanese army of conspiring with forces loyal to President Bashar al-Assad and Hezbollah, which has sent fighters into Syria to support Assad fight a Sunni-led revolt.
The attack hit just hours after a speech by Hezbollah leader Hassan Nasrallah, who said he was protecting Lebanon by fighting in Syria against Sunni fighters.
On Thursday, the Lebanese army killed a suspected Sunni bomb-maker as they raided his house in Arsal.
On Saturday, the army made further raids in Arsal, where they have set up checkpoints this month.
Lebanon has endured rocket and bomb attacks as the Syrian conflict spreads beyond its border, deepening sectarian rivalry in the country, which suffered its own civil war from 1975 to 1990.