The Lebanese Shia militant group, allied with Syrian President Bashar al-Assad, helped regime forces storm the strategically vital western city of about 40,000 residents on Sunday.
"Reliable sources reported to the Syrian Observatory for Human Rights (SOHR) that 23 members of Hezbollah were killed and more than 70 wounded during clashes yesterday in Qusayr city," the pro-opposition, UK-based watchdog group said on its Facebook page.
Syrian activists say Qusayr is suffering some of the heaviest shelling they've seen, with about 15 shells an hour striking the city, as government troops attack from all directions.
"Yesterday was the most violent, most difficult day in the whole of the Syrian revolution," opposition activist Hadi al-Abdullah told Agence France-Presse. "I've never seen so much shelling... Qusayr was being shelled from all sides."
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The Syrian National Coalition, an umbrella organization for opposition groups, warned of a possible "civilian massacre," asking "the international community to take responsibility for protecting the lives of the 40,000 civilians in Qusayr."
The state-run Syria Arab News Agency (SANA) claimed Assad forces had seized and secured a sizable amount of territory from the rebels. "Army units restored stability and security to the full eastern area of Qusayr city in Homs countryside after killing big numbers of terrorists and destroying their hideouts," SANA wrote.
But opposition forces claim they repelled the assault, destroyed at least four tanks and five Hezbollah vehicles, and pushed back most of Assad's forces.
Tareq Murei, an opposition activist, told Reuters Assad's forces had "made incursions into Qusayr but they are now basically back to where they started at the security compounds in east Qusayr and at a ... roadblock to the south."
Qusayr, a strategically important town, allows rebels to access and control a supply route that goes from the Lebanese border to Homs.
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