BEIRUT - Several fighters were killed in an overnight clash between Hezbollah fighters and Syrian rebel forces in Lebanon's eastern border region with Syria, Lebanese security sources said on Sunday. 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 neighbor, 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. The United Nations said on Saturday that up to 1,500 wounded people might be trapped inside Qusair and warned all sides that they would be held accountable for the suffering of civilians. But Security Council diplomats said Russia blocked a council declaration of alarm over the two-week siege of Qusair, 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". It appealed to Assad's government "to allow immediate, full and unimpeded access to impartial humanitarian actors, including U.N. agencies, to reach civilians trapped in al-Qusair". 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 on Syria.