As fierce clashes continue in the border villages of al-Qusayr, there are growing fears that Lebanon is being drawn further into the Syrian conflict.
For more than a week, Syrian rebels have fired shells hitting the eastern Lebanese Hermel region, a bastion for the Lebanese Shia movement Hezbollah, in what they say is retaliation for strikes by the group.
On Tuesday, two new mortar rounds hit the city of Hermel, causing light damage to a house.
Rebels have threatened to "move the battle into Lebanon" if the Syrian government offensive, which they described as Hezbollah-led, continues.
Hezbollah has said those fighting are Lebanese party members who have lived in Syrian border villages for decades and are defending themselves against "rebel attacks".
On Monday, a Hezbollah official described the group's actions as "a national and moral duty in the defence of the Lebanese in border villages".
Hezbollah's involvement in Syria's spiralling conflict has been condemned by the Syrian opposition, which views it as a "declaration of war", and by Lebanese opponents of Hezbollah.
Today, everyone recognises the danger posed by the intervention of [Hezbollah chief Hassan] Nasrallah and his shabiha [pro-Assad militia] in Syria.
Meanwhile, sheikh Ahmad al-Assir, a controversial Lebanese Salafist, has urged his followers to join Syrian rebels fighting troops loyal to President Bashar al-Assad and 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 al-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 was "especially a duty for the Lebanese because Lebanon provides the only gateway" into central Syria.
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."
In the north Lebanese city of Tripoli, a second Sunni sheikh, Salem al-Rafii, echoed Assir's call.
In-depth coverage of escalating violence across Syria
"As Hezbollah sends fighters to defend Shia 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.
On Monday, Rafii called on those who wished to fight in Syria to sign up to join the fight.
Lebanon has adopted an official stance of neutrality in Syria's raging war, which the UN says has left more than 70,000 dead.