One source told the AFP news agency that a suicide bomber had targeted a meeting of Syrian Sunni Muslim clerics.
Arsal, a Sunni town sandwiched between Syrian government-held territory and mainly Shia Lebanese areas sympathetic to it, hosts many Syrian refugees.
Last year, it was the scene of fierce fighting between Lebanese security forces and jihadist militants from the al-Qaeda-affiliated al-Nusra Front and Islamic State (IS).
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