A roadside bomb apparently targeting two vehicles bound for Syria detonated on Monday near Lebanon's main crossing with its war-torn neighbour, a security source told AFP.
"An explosive device detonated on the side of the Taanayel road leading to Masnaa border crossing with Syria," the security official told AFP.
"It targeted a van and a BMW car, but it is unclear who was inside them," the source added.
The source said the two vehicles were headed to Syria and continued in that direction despite the blast.
There were no immediate reports of injuries in the explosion, which an AFP correspondent said could be heard throughout the region, some 14 kilometres (eight miles) from the Lebanon-Syria border.
The blast damaged at least one other car nearby at the time, the source said.
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The conflict in Syria, which began in March 2011, has increasingly spilled across into Lebanon, with tensions rising after Lebanese Shiite group Hezbollah joined the fighting.
The group, which is allied with the Syrian regime, has acknowledged that its fighters are battling alongside Syrian army troops to put down an uprising against President Bashar al-Assad.
In response, Syrian rebels have fired rockets into Hezbollah strongholds along the border.
On Sunday, a demonstrator protesting against Hezbollah's involvement in Syria was killed outside the Iranian embassy in Beirut.
The victim, from a Shiite group opposed to Hezbollah, was shot in the back, an official told AFP.
An AFP photographer at the scene said the young anti-Hezbollah protesters were attacked by party supporters wielding sticks, who also assaulted journalists in the area.