Syria's main opposition National Coalition called on Lebanon to control its frontiers, after rebels said they fired across the border in retaliation against the powerful Shiite movement Hezbollah.
"The Syrian Coalition calls on the Lebanese government to exert control over its borders and put an immediate stop to Hezbollah's military operations on Syrian territory," the group said late Monday.
"We call upon the Lebanese government to take action against Hezbollah's aggressions and do everything within their means to ensure the safety of the innocent civilians on the Syrian-Lebanese border," it said in a statement.
"For weeks now, forces belonging to Hezbollah have targeted villages inside Syria, located on the border of Syria and Lebanon. Hezbollah deployed forces into some border villages and took control of those areas.
"The (rebel) Free Syrian Army was forced to respond to these repeated aggressions," it said.
The statement comes after Lebanon said it would submit to the Arab League a letter of protest condemning the spillover of fire from Syria onto its territory.
The Coalition is recognised by dozens of states and organisations -- among them the Arab League -- as the legitimate representative of the Syrian people.
Beirut has adopted a policy of "disassociation" in the conflict in neighbouring Syria, and has been reluctant to publicly blame either rebel or regime forces.
The rebels claimed to have fired shells on Lebanon at the weekend, blaming Hezbollah for firing from Lebanon and positions inside Syria on rebel-held areas in the strife-torn Qusayr area, near the border.
A rebel commander told AFP on Monday they were "giving the Lebanese authorities an opportunity to respond, to take practical steps to put a stop to (Hezbollah's) shelling", while threatening to launch new attacks should the Shiite group continue to target rebels in Syria.
Though 30 years of political and military domination by Damascus over Lebanon ended in 2005, Syria's regime has continued through its Hezbollah-led allies to exert significant influence over its smaller neighbour.
Lebanon is sharply divided over Syria's two-year conflict, with Hezbollah and its allies supporting President Bashar al-Assad, and the Sunni-led March 14 movement backing the rebels.
Cross-border shellfire from the Syrian war has regularly hit Lebanon, on occasion killing Lebanese. On Sunday, however, two people died for the first time in Hezbollah strongholds of the border region.