One source said 15 rebels were killed in the fighting east of the Bekaa Valley town of Baalbek, but the exact toll would not be clear until bodies could be retrieved from the remote and rugged border area. One Hezbollah fighter also died, he said.
Syria's two-year-old conflict has increasingly sucked in its smaller neighbour, with deadly fighting shaking the northern Lebanese city of Tripoli and rockets hitting the Bekaa Valley and southern Beirut.
Guerrillas from Shi'ite Muslim Hezbollah, which supports the Iranian-backed President Bashar al-Assad, are fighting alongside his army to drive rebels from the Syrian border town of Qusair, while Lebanese Sunni Muslim fighters have joined the anti-Assad revolt.
The latest fighting took place near Ain el-Jaouze in a finger of Lebanese territory which extends into Syria. The sources said the rebels may have been ambushed as they set up rockets to fire into Shi'ite areas of the Bekaa Valley.
Rebels have said they will carry out attacks inside Lebanon in response to Hezbollah's support for Assad's assault on Qusair, a strategic town for rebel weapons supplies and fighters coming into Syria from Lebanon.
But Security Council diplomats said Russia blocked a council declaration of alarm over the two-week siege ofQusair, arguing that the council had done nothing when the town was first taken over by anti-Assad fighters.
The draft statement also urged forces loyal to Assad and rebels trying to oust him "to do their utmost to avoid civilian casualties and for the Syrian Government to exercise its responsibility to protect civilians".
Moscow's move to block the statement highlights the deep chasm between Russia and Western nations on how to deal with the war in Syria despite joint efforts by Washington and Moscow to convene a peace conference onSyria.
French Foreign Minister Laurent Fabius suggested on Sunday that the conference could take place in July. He said the Syrian government and the opposition must attend what he called "the last chance" for a negotiated solution.
"It's not just about getting round the table and then asking what are we going to talk about. It needs to be prepared. That is why I say that the July date would be suitable," Fabius said.
Assad has lost control of large areas of northern and eastern Syria but his forces have been fighting fierce counter-offensives in the south and centre of the country, including Damascus, Deraa and Qusair.
The fighting has strengthened Assad's hand ahead of the proposed peace talks, which the 47-year-old leader says he supports in principle. However he has dampened prospects of any transfer of his powers to a transitional government - a central element of efforts to secure a political solution. Assad's opponents have also yet to commit to the peace talks.
The uprising against Assad, from the Alawite minority which is an offshoot of Shi'ite Islam, has killed at least 80,000 people, driven 1.5 million refugees across its borders and fuelled regional sectarian tensions.
Leading Sunni Muslim cleric Sheikh Youssef al-Qaradawi, based in Qatar which has led regional pressure for Assad's overthrow, called on Saturday for holy war against the Syrian government after intervention by Hezbollah, whose name in Arabic means the Party of God.
(Reporting by Dominic Evans; Editing by Angus MacSwan)