Sheikh Ahmad al-Assir, a controversial Lebanese Salafist sheikh, has urged his followers to join Syrian rebels fighting troops loyal to President Bashar al-Assad and Lebanese Shiite movement Hezbollah.
The call came as a second Sunni Lebanese sheikh called the fight against Assad's regime a "jihadist duty."
"Today, everyone recognises the danger posed by the intervention of (Hezbollah chief Hassan) Nasrallah and his shabiha (pro-Assad militia) in Syria," Assir, who is based in the southern Lebanese city of Sidon, told his followers late Monday.
Syria's opposition and monitoring groups have accused Iran-backed Hezbollah of sending elite fighters to battle alongside regime troops in Qusayr, an area of Syria's central Homs province near the Lebanese border.
"Nasrallah and his shabiha have taken the decision to enter into these areas (Qusayr) in order to massacre the oppressed people there," Assir added.
"There is a religious duty on every Muslim who is able to do so... to enter into Syria in order to defend its people, its mosques and religious shrines, especially in Qusayr and Homs."
Assir said joining the fight in Homs is "especially a duty for the Lebanese because Lebanon provides the only gateway" into central Syria.
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He said his address mainly targeted "residents of the border areas," but added: "This fatwa (religious decree) affects us all, especially those who have military experience."
Assir has seen his following swell in the last year, in part due to his firebrand speeches and staunch opposition to Hezbollah, the Shiite Lebanese movement allied with Assad's regime.
His popularity has grown amid a crisis in traditional Sunni Lebanese political groups, and a growing sectarian backlash from the conflict in neighbouring Syria.
Though Syria's revolt began as a peaceful uprising, it has developed into an increasingly sectarian conflict, particularly in the province of Homs.
The majority of Syria's rebels, like the population, are Sunni, while Assad come from the minority Alawite community, an offshoot of Shiite Islam.
In his Monday speech, Assir also announced the establishment of "free resistance battalions" in Sidon, where he is based.
"Allahu Akbar (God is greatest)! Death or humiliation!" his followers chanted.
Meanwhile, in the north Lebanese city of Tripoli, a second Sunni sheikh, Salem al-Rafii, echoed Assir's call.
"As Hezbollah sends fighters to defend Shiite areas... we will also send money and men to our Sunni brothers in Qusayr," he said.
"We also call on all young Sunnis to be ready, as a first wave of young men and weapons will be sent to carry out their jihadist duty in Qusayr and to defend Sunni regions," he added.