Syrian rebels and Hezbollah 'exchange fire in Lebanon
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Lebanese security sources said the clashes took place on Lebanon's side of the border, near the town of Baalbek.
Hezbollah is fighting alongside the army in Syria, but the clashes have rarely crossed onto Lebanese soil.
Meanwhile the Red Cross has said it is alarmed by the worsening situation in the besieged Syrian town of Qusair.
It has appealed for access to the town, which lies just 10km (6 miles) over the Lebanese border.
Thousands of civilians are believed to be trapped as pro-government forces - including Hezbollah fighters - battle rebels.
The office of UN Secretary General Ban Ki-moon also appealed to the warring parties to allow residents to flee.
Mr Ban telephoned Syrian Foreign Minister Walid al-Moualem to share his concerns, but was told the Red Cross would be allowed in "as soon as military operations are over", Reuters reports.
Mr Moualem reportedly expressed his surprise at the level of concern over Qusair, saying there had been none when rebels took over the town last year.
Also on Sunday, a car bomb killed nine members of the Syrian security forces in the capital, Damascus, according to the British-based pro-opposition watchdog the Syrian Observatory for Human Rights.
The watchdog said the bomb had targeted a police station in the eastern district of Jobar, which has seen renewed clashes between government forces and rebels who are entrenched there.
Qusair 'blockade'
Casualty reports from the Baalbek clashes, just over the border from Syria, have not been independently verified, but Reuters news agency quoted a Lebanese security source on Sunday as saying that at least 15 people were killed.
There were reports that at least one Hezbollah fighter was among the dead.
Lebanese media quoted officials as saying the rebels had been preparing to launch rockets when the clashes broke out.
The Syrian rebels have threatened Hezbollah targets in Lebanon in revenge for its backing of the government in Damascus, and have frequently fired rockets into Lebanon, including several on Saturday.
The BBC's Jim Muir in Beirut says the reported clashes are another a symptom of the possible dangerous spread of the Syria conflict.
Hezbollah, a Shia movement, has stirred up sectarian feelings in Lebanon by giving support to the Syrian government.
In addition, some Lebanese Sunnis have also crossed into Syria to fight alongside the rebels, who are drawn largely from Syria's majority Sunni community, meaning Lebanon's domestic conflict has effectively drifted over into Syria, says our correspondent.
Hezbollah fighters are also involved in the siege of Quasir, which is considered a key logistical hub and supply route for weapons smuggled into Syria.
An opposition activist told the BBC on Friday that around 30,000 civilians were still in the town, effectively under blockade.
The BBC's Imogen Foulkes in Geneva says the fact that both the UN and ICRC have issued urgent statements on Qusair at the same time is an indication of how desperate they believe the situation there has become.
The UN Security Council attempted to issue a declaration voicing "grave concern" about Qusair, but it was blocked by Russia so failed to obtain the necessary unanimous agreement of council members.
A diplomat said Russia blocked the draft text because the UN had failed to speak out when Qusair was seized by rebels.
Fighting in Qusair intensified last month with militants from Hezbollah joining forces loyal to President Bashar al-Assad.
Reinforcements from the rebel Free Syrian Army are reported to have managed to break through from the north-east to support the embattled rebel fighters.
Some Lebanese Sunnis have also crossed into Syria to fight alongside the rebels, who are drawn largely from Syria's majority Sunni community.
Activists from the Syrian Observatory for Human Rights say rebels in Qusair are bracing themselves for another assault.
Fifteen Syrian army tanks have massed north of the town, says Rami Abdel Rahman, the observatory's director.
"Regime forces are reinforcing the sites that they have north of the city, including Dabaa airport and Jawadiya," he said.
The UN estimates that than 80,000 people have been killed and 1.5 million have fled Syria since the uprising against President Bashar al-Assad began in 2011.
