The head of al-Qaeda's branch in Syria, al-Nusra Front, has warned that the "real battle" in neighbouring Lebanon has yet to begin, indicating that more of the violence gripping Syria will spill across the border.
In the audio message distributed online on Tuesday night, Abu Mohamed al-Jolani warned that Lebanon's Shia Hezbollah movement would regret its intervention in the Syrian conflict in support of President Bashar al-Assad's regime.
The threat came a day after Hassan Nasrallah, head of Shia Hezbollah, declared "victory" for his movement against self-declared jihadists in Syria, as tens of thousands of Shia Muslims gathered in Beirut to commemorate Ashoura.
Referring to a mountainous region that runs along the Syria-Lebanon border, Jolani said: "Our brothers in Qalamun are hiding plenty of surprises.
"The real battle in Lebanon has yet to begin," he added, saying Nasrallah would "regret what he has done to Sunnis in Syria."
Hezbollah's decision to send fighters to bolster Assad's regime has drawn the ire of moderate and radical groups in Syria alike.
Al-Nusra and other extreme groups have actively targeted the group's strongholds in Lebanon in a series of bomb attacks that have killed dozens.
Lebanon has seen the conflict next door spill over increasingly, with tensions rising between its Sunni community, which largely backs the Syrian uprising, and Shia who back the Syrian government of Alawite al-Assad.
In addition to the bomb attacks, armed groups like the Sunni Al-Nusra and the Islamic State in Iraq and Levant have battled Lebanese troops and Hezbollah forces along the border.
In August, fighters from the two armed groups briefly overran the Lebanese border town of Arsal, and they are still holding 27 Lebanese police and soldiers they took hostage, having executed three of them.
Nearly 190,000 people have been killed in Syria since the conflict erupted in March 2011.