Syria's opposition chief laid down conditions on Wednesday for talks with President Bashar al-Assad's regime, as world outrage at a massacre piled pressure on all parties to halt their 22-month conflict.
And amid concerns that Syria's stockpile of chemical weapons could fall into the hands of Lebanon's Shiite militia Hezbollah, Israeli forces carried out an air strike on a convoy near the Syria-Lebanese border, security sources told AFP.
Syrian National Coalition leader Moaz al-Khatib's surprise move came after UN envoy Lakhdar Brahimi said the war had reached "unprecedented levels of horror," and UN chief Ban Ki-moon warned of a "catastrophic" situation in Syria.
Khatib said via his Facebook page he is ready for dialogue with officials of Assad's regime subject to conditions, including that "160,000 detainees" are released and that passports for exiled citizens be renewed in embassies abroad.
Until now, Syria's main opposition groups have said they are only prepared to enter into dialogue on ways to end the 22-month conflict if Assad steps down.
"I announce I am ready for direct discussions with representatives of the Syrian regime in Cairo, Tunis or Istanbul," Khatib said.
"I became aware thanks to the media that the regime in Syria has called on the opposition to enter into dialogue," said Khatib, who heads the main Syrian opposition umbrella group.
"While it is not right for anyone to bargain with the freedom for which our people have paid so dearly in blood, I say there are basic conditions before I sit down with representatives of the regime," he said.
But he added that "we cannot trust a regime that kills children, attacks bakeries bombards universities, destroys Syria's infrastructure and massacres innocent people."
Assad proposed earlier this month a national dialogue to end Syria's crisis, but he made it clear this would only apply to groups not linked to the armed insurgency, effectively shutting out the National Coalition.
Khatib said the unprecedented statement expressed his own opinion only and that his group would meet on Thursday to discuss the proposal.
An influential opposition group, the Syrian National Council (SNC) -- a key component of the Coalition -- swiftly rejected the proposal.
"The Syrian people have -- and are still paying -- an extremely high price for their full freedom and to get rid of every last remnant of this oppressive, tyrannical regime," said the SNC, adding that it "rejects any settlement or negotiation with the Syrian regime."
The Coalition had earlier Wednesday lashed out at the "global inaction" which it said was giving Assad's regime a licence to kill, accusing Assad's forces of being behind the killings of at least 78 people whose bodies were Tuesday in a river in northern Syria's Aleppo.
Witnesses and activists said all the victims had been executed with a single gunshot to the head or neck.
The Syrian authorities accused the jihadist group Al-Nusra Front of having carried out the horrifying massacre, but rebels and residents in the area said they had no doubt Assad's forces were behind the killings.
The Israeli air strike came as UN envoy Lakhdar Brahimi issued a fresh warning of the conflict spilling over into neighbouring countries. "None of the neighbors is immune to the fallout consequences of the conflict," he told the UN Security Council.
Sources who knew of the strike differed on whether it occurred on Syrian or Lebanese territory.
"The Israeli air force blew up a convoy that had just crossed the border from Syria into Lebanon," one source said, adding that the convoy was believed to be carrying weapons, without specifying the type.
A second security source, speaking on condition of anonymity due to the sensitivity of the issue, also confirmed to AFP that Israeli warplanes had hit a convoy allegedly carrying weapons to Lebanon but said the incident occurred just inside Syria.
The attack came after Israel expressed concerns that Damascus's stockpile of chemical weapons could fall into the hands of Hezbollah, an ally of the Syrian regime, or other militant groups.
In his remarks to the Security Council on Tuesday, Brahimi said Syria "is breaking up before everyone's eyes."
UN chief Ban Ki-moon issued an equally dire assessment, telling an international donors conference in Kuwait that the situation in Syria "is catastrophic", with half the hospitals and a quarter of the schools destroyed and vital infrastructure badly damaged.
At the end of the one-day conference, Ban announced that more than the targeted $1.5 billion in aid for stricken Syrians had been pledged.