Sunni rebel fighters killed some 60 Shiites in a fresh outbreak of sectarian violence in the Syria conflict, which on Wednesday takes centre stage in US-British talks in Washington.
In the latest spillover of the civil war, a Syrian helicopter gunship fired on the Lebanese town of Arsal, whose mainly Sunni residents support the insurgency against Syrian President Bashar al-Assad.
A Lebanese security official told AFP that three missiles slammed into the heart of Arsal, wounding several people.
A week on from fierce battles between Syrian soldiers and rebel fighters near the Israeli-occupied Golan Heights, 378 Austrian peacekeepers deployed to monitor the ceasefire zone began to pull out from the area.
And in a new macabre episode of the war, Sunni Islamist rebels celebrated the killing of some 60 Shiites, mostly pro-regime fighters, according to amateur video distributed by the Syrian Observatory for Human Rights.
"Look Shiites, this is how you will end up, you dogs," cries one man shown in the footage filmed in the village of Hatlah, in the eastern province of Deir Ezzor.
"Sunnis, help your community," says another man wearing a black bandana inscribed with the Islamic profession of faith.
"God is greatest. All the Shiite houses have been burned down... Look at the fighters of the jihad (holy war) celebrating their entry into the Shiite infidels' houses," says a man filming a second video.
The rebels assaulted Hatlah in retaliation after Shiite fighters from the village attacked a rebel position, killing two insurgents, said the Observatory.
The majority of Syria's population is Sunni but has been ruled by more than 40 years by the Assad clan, who belong to the Alawite community, an offshoot of Shiite Islam.
Hezbollah's support for the Assad regime has further heightened sectarian hatred in Syria, especially after rebels lost the central town of Qusayr to government forces aided by fighters from the Lebanese Shiite movement last week.
Boosted by their victory in Qusayr, regime troops advanced on parts of the nearby city of Homs, as they prepared an assault on Aleppo in the north, the Observatory said.
France meanwhile urged the international community to stop the Assad regime.
"We need to re-balance things because over the past few weeks the troops of Bashar al-Assad and especially Hezbollah and the Iranians, along with Russian arms, have gained considerable ground," said Foreign Minister Laurent Fabius.
But he did not expand on how Syrian troops, buoyed by military support from its Shiite allies Hezbollah and Iran, should be stopped.
A day earlier, Fabius said: "Bashar... used chemical weapons in an outrageous manner. We must stop him because, if there is no re-balancing on the ground, there will be no peace conference in Geneva as the opposition will refuse to come."
A US-Russian peace initiative that would bring regime and rebel representatives to the negotiating table appears stalled amid the regime's advances on the battlefield.
No date has been set for such a conference to take place, almost 27 months into a conflict that the Observatory says has killed more than 94,000 people.
In Washington, Secretary of State John Kerry was set to meet his British counterpart William Hague, a day after President Barack Obama asked his security team to "look at all options" to help the opposition.
The US State Department said there would be no American "boots on the ground".
Britain's Prime Minister David Cameron announced staunch Assad backer Russian President Vladimir Putin would fly to London for talks on Syria next week.
The announcement comes a day after Putin said he believed Assad should have implemented political reforms that could have averted the current bloodbath.
Syria's regime has consistently refused to recognise the existence of any popular uprising against Assad, blaming all violence on foreign-backed "terrorists".
At least 257 people were reported to have been killed across the country on Tuesday, including the Shiites slain in clashes in the eastern village of Hatlah.