One of the Arab world's most influential Sunni clerics, Yusuf al-Qaradawi, urged the faithful this week to fight alongside Sunni rebels against Shiite Hezbollah and President Bashar Assad's minority Alawite sect, an off-shoot of Shiite Islam.
Hezbollah's involvement in the battle over strategic Qusair has also raised tensions with Syrian rebels who have threatened to target the militia's bases in Lebanon, and with Sunnis in Lebanon who support the rebels.
Clashes between Sunnis and Alawites erupted Sunday evening in Lebanon's northern city of Tripoli, wounding at least 14 people, according to the state-run National News Agency.
Also Sunday, three rockets from Syria struck northeastern Lebanon, a day after 18 rockets and mortar rounds hit Lebanon's eastern Baalbek region, a Hezbollah stronghold.
From Saturday night into Sunday, Hezbollah encircled and ambushed Syrian rebels and allied Lebanese fighters whom they suspected of rocketing Baalbek, a Lebanese security official said.
A Hezbollah fighter and several rebels were killed in the clashes in a remote area near the Syrian border, said the official, who spoke on condition of anonymity in line with regulations.
The Lebanese TV station Al-Mayadeen, seen as sympathetic to the Syrian regime, quoted Lebanese security officials as saying 17 fighters from Jabhat al-Nusra, a rebel group linked to the global al-Qaida terror network, were killed in the fighting. The report could not be independently confirmed.
Hezbollah chief Hassan Nasrallah has linked his militia's fate to the survival of Assad's regime, but pledged in a televised speech last month that he would keep the battle out of Lebanon.
Hezbollah is the most dominant faction in Lebanon's patchwork of ethnic and religious groups. A backlash against Hezbollah's involvement in Syria and a creeping destabilization of Lebanon could hurt the group's standing at home.
Events in Lebanon could spin out of control, even if rival Lebanese groups don't want Syria's war to be exported to Lebanon, said Hilal Khashan, a political science professor at the American University of Beirut. With Lebanese fighters increasingly engaged on opposite sides in Syria, "the worst is yet to come" in Lebanon, he said.
Earlier this week, Lebanon's parliament put off general elections scheduled for this month for another 17 months, citing a deteriorating security situation.
Syrian activists reported new fighting in Qusair, about six miles from the Lebanese border. Local activist Hadi Abdullah, speaking by Skype, reported heavy shelling and regime airstrikes on the town, saying at least four people were killed and more than 30 wounded.
He said thousands of civilians were trapped in Qusair, including hundreds of wounded. Hezbollah cut off running water when it seized the water station, he said, and food was running out.
The Britain-based Syrian Observatory for Human Rights reported at least three killed in Qusair on Sunday.
The International Committee of the Red Cross and U.N. humanitarian agencies expressed alarm over the fate of thousands of civilians believed trapped in Qusair, including many wounded.
They called on both sides to allow aid to reach civilians, including the wounded. The Red Cross said many of the wounded were not receiving the medical care they need and that food, water and medical supplies were scarce. The U.N. agencies called for an immediate cease-fire to allow civilians to leave the town.
On Sunday, U.N. chief Ban Ki-moon called Syrian Foreign Minister Walid al-Moallem to express concern over the situation in Qusair, according to Syria's state-run news agency SANA.
Al-Moallem told the U.N. chief that the Red Cross and other aid agencies would only be able to enter Qusair until after the end of military operations there, SANA said.
So far, neither side has been able to deliver a decisive blow in Qusair.
A regime victory would solidify Assad's control over the central province of Homs, the linchpin linking the capital Damascus with the Alawite strongholds on the Mediterranean coast. For the rebels, holding the town means protecting their supply line to Lebanon.
The town has become emblematic for the state of Syria's civil war recent military gains by the regime, but not enough to dislodge the rebels completely, and inaction by a divided international community.
Last month, the U.S. and Russia, who are on opposite sides of the conflict, sought to revive the idea of peace talks between the regime and Syria's political opposition. But prospects for launching talks at an international conference in Geneva began fading after the main Western-backed opposition group said it wouldn't attend as long as Hezbollah fights in Syria and the situation in Qusair remains dire.
Meanwhile, Pope Francis called Sunday for prayers for "beloved Syria" as he spoke to a crowd in St. Peter's Square at the Vatican. He lamented that the war has stricken a defenseless people aspiring to peace, and decried the "plague of kidnappings" in Syria.
Both rebels and pro-regime forces have abducted political foes, members of rival sects and others, including journalists, to settle scores or for ransom. Among those snatched were two Orthodox bishops who were abducted in April. The pope assured families of hostages of his prayers.