NABI CHIT, Lebanon — At the entrance to this village in Hezbollah’s Bekaa Valley heartland, under a sign welcoming visitors to “The Citadel of Resistance,” workers on Monday hoisted a freshly printed banner honoring a young man described as one of Hezbollah’s latest martyrs — killed in battle not with Israel, the foe the group’s guerrillas train to fight, but with Syrian rebels.

Down the road, another dead fighter’s uncle, Fayez Shukor, welcomed mourners under a tent overlooking the valley as the sun set on a day that had seen Hezbollah’s death toll rise to unexpected heights as the group joined Syrian forces trying to storm the rebel-held Syrian city of Qusayr. His nephew, he had said earlier, died on Sunday alongside 11 other Hezbollah fighters killed in a single rebel attack.

Lebanon reeled Monday from the twin realizations that Hezbollah, the nation’s most powerful military and political organization, was plunging deeper into a war the country has tried to stay out of, and that the group was taking unaccustomed losses. Mr. Shukor, a former government minister from Lebanon’s Arab Socialist Baath Party, walked a careful line between supporting a declaration by Hezbollah that Syria’s fight is its fight and acknowledging the contradiction of fighting fellow Arab Muslims instead of Israelis.

“I wish all this blood had been shed in the south, fighting Israel,” Mr. Shukor said, but added that the rebels battling Hezbollah’s ally, President Bashar al-Assad of Syria, were “infidels and garbage” serving Israel; the West, he said, should recognize that they are Al Qaeda-linked extremists and help wipe them out.