"The number of Lebanese Hezbollah fighters killed in recent months in the outskirts of Damascus and Homs has risen to 141," the Syrian Observatory for Human Rights said in a statement.
"That includes 79 fighters killed from the period starting at dawn on May 19 to dawn yesterday, killed by mines, snipers and fighting in the town of Qusayr and its surroundings," the group added.
Yesterday, a source close to Hezbollah said the group's toll in several months of fighting was 110, most of them killed in Qusayr.
But the battle to retake rebel stronghold Qusayr in the central province of Homs has been its biggest and bloodiest engagement so far.
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On Saturday alone, 22 fighters from the Shiite group were killed in Qusayr, the source close to the group told AFP.
The same day, Hezbollah chief Hassan Nasrallah vowed his group would win a "victory" in Syria.
"I say to all the honourable people, to the mujahedeen, to the heroes: I have always promised you a victory and now I pledge to you a new one" in Syria, he said at a ceremony marking the 13th anniversary of Israel's military withdrawal from Lebanon.