Seventy-five fighters from Lebanon's Hezbollah have been killed in Syria since late last year, a source close to the Shiite militant group said on Thursday.
The Syrian Observatory for Human Rights gave a higher death toll, saying 104 Hezbollah members had been killed in Syria since last autumn, but a Hezbollah spokesman denied the figures.
"There have been 57 killed and 18 others who have died of their wounds since the start of its participation in the war in Syria," the source close to Hezbollah told AFP.
Observatory director Rami Abdel Rahman said 104 Hezbollah fighters had been killed in all in fighting in central Homs province, which borders Lebanon, and around a revered Shiite pilgrimage site just south of Damascus.
"In the past five days, 46 were killed in Qusayr, 20 more died in the same area earlier this month, and 38 have died since the autumn in Homs province and at the Sayyeda Zeinab shrine," the Britain-based watchdog chief said.
Hezbollah spokesman Ibrahim Musawi told AFP: "I deny these figures. When we decide to give any information, we'll be in touch."
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Hezbollah combatants have become increasingly involved in Syria's conflict, fighting alongside President Bashar al-Assad's forces against an insurgency that flared after a brutal regime crackdown on democracy protests.
Initially Hezbollah said it wanted only to defend 13 Syrian villages along the border where Lebanese Shiites live, and the Sayyeda Zeinab shrine, revered by Shiites around the world.
However its elite fighters later encircled the rebel-held central town of Qusayr with regime troops before the launch on Sunday of a withering assault on the strategic border town that is home to 25,000 people.
Hezbollah denied its involvement in Syria for some time, quietly burying fighters killed in the fighting there.
But the movement stopped hiding its dead when its leader Hassan Nasrallah on April 30 paid homage to militants killed across the border.
"Syria has true friends in the region who will not allow Syria to fall into the hands of the United States, Israel and 'takfiri' groups," he said in an interview with the movement's television channel, Al-Manar.
Waddah Sharara, an expert on the organisation, says Hezbollah has some 20,000 fighters, of whom 5,000 to 7,000 have combat experience, and between 800 and 1,200 of them have been fighting at Qusayr.