Western officials will meet the commander of the main force fighting President Bashar al-Assad on Saturday to discuss new aid, diplomats said, signalling a new bid to help mainstream rebels and counter the strength of sectarian militias.
Underscoring the West's fears of an uncontrolled sectarian bloodbath, activists said Sunni insurgent militiamen had killed some 60 Shi'ites in a town in the east of Syria.
Western countries have shown more determination in recent weeks to play an active role in Syria's civil war after largely sitting on the sidelines while calling for Assad's downfall during two years of fighting in which 80,000 people were killed.
The situation on the ground has changed dramatically in recent weeks, with Assad winning the open support of hardened fighters from Hezbollah, the Shi'ite militia from neighbouring Lebanon, which helped his forces take a strategic town.
Meanwhile, the West has been alarmed by the rise of groups like the al-Nusra Front, which has pledged loyalty to al-Qaeda, among the mainly Sunni rebels.
France and Britain want to counter the influence of sectarians on both sides by providing more support to moderate rebels to help end the war and bring down Assad.
US Secretary of State John Kerry said Washington was discussing new action it could take to aid the rebels, although it was not yet ready to make any announcements.
"We are determined to do everything that we can in order to help the opposition ... to save Syria," he said at a news conference with British Foreign Secretary William Hague, who was visiting Washington.
Western countries have so far refused to send arms to the rebels directly, although they have provided support for Arab states like Qatar and Saudi Arabia that do. And this month, France and Britain forced the European Union to lift an arms embargo, clearing the way to start shipping arms.
Western diplomats said representatives would be meeting Free Syrian Army commander Salim Idriss on Saturday in Northern Turkey to discuss possible new aid.
"Idriss needs money, munitions and weapons to cement his leadership and win credibility among the fighters," one diplomat said, speaking on condition of anonymity as the meeting with Idriss has not been publicly announced.
Western countries hope that by channelling assistance through Idriss, a former senior commander in Assad's army, they can reduce the influence of groups like al-Nusra, which Washington views as a front for Sunni al-Qaeda fighters from Iraq.
The Syrian Observatory for Human Rights, a pro-opposition monitoring group which has documented killings by both sides, said Sunni militants had killed about 60 Shi'ites in the rebel-held eastern town of Hatla.
A video posted online by rebels on Tuesday, entitled "The storming and cleansing of Hatla" showed dozens of gunmen carrying black Islamist flags celebrating and firing guns in the streets of a small town as smoke curled above several buildings.
"We have raised the banner 'There is no God but God' above the houses of the apostate rejectionists, the Shi'ites, and the holy warriors are celebrating," the voice of the cameraman says.
"This is a Sunni area, it does not belong to other groups," one fighter shouted in the video.
The Observatory said most of the dead were pro-Assad Shi'ite militiamen, as Shi'ite civilians had fled. It was not possible to independently verify the account.
The sense of the Syrian fight being a battle between faiths was taken up a notch after Shi'ite guerrillas from Lebanon's Hezbollah helped Assad's forces take the rebel stronghold of Qusair last week. Some fighters in Hatla can be heard in the video calling the attack "the first revenge for Qusair".
Activist Karam Badran, who spoke to Reuters from nearby Deir al-Zor, said only 20 people had been confirmed killed in Hatla and another 20 had been taken hostage by the rebels.
"Three of the men killed were three Shi'ite clerics. They were executed and hung on the gates of the town," Badran said, adding that the dead also included Sunnis who had joined pro-Assad militia.
Separately, in the Damascus area, rebels reported that 27 of their comrades had been killed in an ambush near the town of al-Maraj. Video uploaded by activists showed victims shot in the face or head. The camera scanned over several bloodied and dirt-coated corpses as men called out for help to wash the bodies.
Musaab Abu Qutada, an opposition activist, said the men had been trying to break through a military blockade to bring supplies into rebel strongholds in suburbs of the capital.
The balance of power has shifted with the arrival of Hezbollah to fight on behalf of Assad, whose minority Alawite sect is an offshoot of Shi'ite Islam. If six months ago Western countries were predicting Assad would soon be toppled, they now see no military solution to the conflict in sight.
Western officials fear that unless more robust action is taken, Syria will effectively be left divided between Shi'ite Hezbollah and Sunni al Qaeda, with the most ruthless sectarians coming to the forefront on both sides, causing mass carnage like that in Iraq after U.S. troops toppled Saddam Hussein.
The United Nations warned on Wednesday of a humanitarian crisis affecting 1.2 million people in the area around Damascus, saying fighting in the district had made it impossible to bring in desperately needed aid.
The rebels enjoy the backing of Arab states and Turkey as well as the West. Assad is supported by regional Shi'ite power Iran and Russia, which has used its veto to block U.N. Security Council action against him.
Washington and Moscow have jointly called for an international peace conference in Geneva, the first attempt in a year by the powers supporting the opposite sides to resolve the conflict, but hopes are slim of reaching a breakthrough.
Turkish Foreign Minister Ahmet Davutoglu said Assad's government was exploiting the plans for the Geneva meeting to stall, while taking advantage of the intervention by Hezbollah that led to the "ethnic cleansing" of Qusaid.
"We were much more hopeful, but the developments in Qusair have shown that all of these diplomatic manoeuvres buy time for Assad and pave the way for a rise in the massacres by the Assad regime," he said.
(Additional reporting by Humeyra Pamuk in Ankara, Louis Charbonneau at the United Nations, Arshad Mohammed and Lesley Wroughton in Washington; Writing by Peter Graff; Editing by David Brunnstrom)