A Hezbollah commander and several fighters from the Lebanese militant group were killed in Syria, said a Lebanese security official on Tuesday.
The news could escalate tensions over Hezbollah's role in Syria's civil war, said the Associated Press.
Hezbollah's support of the Syrian regime has already battered its reputation, but the AP said any sign of the group's fighters joining the conflict might spark wider conflict.
Hezbollah said Ali Hussein Nassif had been killed "performing his jihadist duty" and was buried on Monday, according to the BBC. Syrian rebels said Nassif and his fighters were killed in an ambush by the Free Syrian Army. Other reports suggested they died in border clashes.
Also on Tuesday, Reuters reported that a pro-Assad Lebanese newspaper said Assad had toured the war-torn city of Aleppo by helicopter and ordered about 30,000 additional troops to the area.
Hezbollah spokesman Ibrahim Moussawi confirmed the deaths of Hezbollah members but did not provide further information on cause or location of death, the AP said.
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The US has accused Hezbollah of "providing training, advice, and extensive logistical support" to the regime of Syrian President Bashar al-Assad.
Lebanese officials have also said Hezbollah's fighters are likely taking part in the conflict, pointing to quiet burials of martyrs as evidence. "Obituaries for Hezbollah fighters have begun to appear in Lebanese newspapers, without the circumstances of the deaths being explained," said the BBC.
Reuters noted it's possible that the Shia Hezbollah have not confirmed their military presence for fear of stoking tensions with Sunni Muslims in Lebanon, many of whom support the Syrian opposition.
However, regime change in Syria to a Sunni-led majority would be a dire outcome for Hezbollah. "Iran remains the group's most important patron, but Syria is a crucial supply route. Without it, Hezbollah will struggle to get money and weapons as easily," said the AP.
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