The group, which relies on a network of activists, doctors and lawyers on the ground, said pro-regime forces carried out a "massacre" using guns and knives to kill residents of Sheikh Hadid village.
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"Fifteen people were killed – two women, a child and 12 men – by regime forces backed by the Popular Committees and National Defense Forces Alawite militias using guns and knives," the group said on Saturday. Ten other people were wounded in the incident, it added.
Syrian President Bashar Assad comes from the Alawite minority community, and is battling a rebellion dominated by Sunni Muslims.
The Observatory said there had been clashes between rebels and regime forces in the village, and it was not clear whether all the men killed were civilians. Rebel forces managed to take control of the nearby village of Jelma, the group added.
Iran 'part of the problem'
Meanwhile, Syria's opposition National Coalition rejected Saturday an Iranian offer to broker talks with the regime, saying the bid was "not serious" and calling Tehran "part of the problem."
"The Iranian initiative is not serious and lacks political credibility," the key opposition grouping said in a statement.
But the Syrian opposition said it was skeptical that Iran, a staunch ally of President Bashar Assad, could broker talks, calling Tehran "part of the problem."
"It would be more useful if the Iranian leadership withdrew its military experts and fighters from Syrian territory," the Coalition said.
The Syrian opposition has long alleged that Iran has sent military personnel to help Assad's forces in their fight against the rebels.
Iran is also a key backer of Lebanon's Shiite Hezbollah movement, which has sent fighters to bolster Syria's armed forces in key battles, notably their recapture of the strategic central town of Qusayr in June.
In addition Tehran has helped bolster the regime by providing funds to the cash-strapped government.
Rohani, a moderate on Iran's political scene, has made several diplomatic overtures since his election in June, and there has been speculation that he could meet US President Barack Obama on the sidelines of the UN General Assembly next week.
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