Syria's 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 umbrella opposition group also called on rebel forces in Syria to "exercise restraint" and to respect Lebanon's sovereignty.
"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," said the Coalition.
"Due to the sensitivity of the situation on the border of Syria and Lebanon, we call on the Free Syrian Army battalions in Homs (a province that borders Lebanon) to exercise restraint and to respect the sovereign borders of Lebanon."
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 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 shelled Lebanon at the weekend in retaliation for what it said was Hezbollah fire from Lebanon and Syria on rebel-held areas in the strife-torn region of Qusayr, 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.