Syria accused Israel of staging an air raid on a military research centre, a day after a peace envoy issued a fresh warning of the 22-month conflict spilling over into neighbouring states.
The strike came as an opposition chief laid down conditions for talks with President Bashar al-Assad's regime, and as world outrage at a massacre piled pressure on all parties to halt their bloodletting.
UN Secretary General Ban Ki-moon warned of a "catastrophic" situation in the war-torn country at a conference in Kuwait where international donors pledged more than the targeted $1.5 billion in aid for stricken Syrians.
The Syrian army accused Israel of launching a dawn strike targeting a military research centre in Jamraya, near Damascus, in a statement carried by state news agency SANA.
"Israeli fighter jets violated our airspace at dawn today and carried out a direct strike on a scientific research centre in charge of raising our level of resistance and self-defence," the general command said.
The warplanes entered Syria's airspace via Mount Hermon, or Jabal el-Sheikh in Arabic, at low altitude and under the radar, the army said, adding that two site workers were killed.
"They... carried out an act of aggression, bombarding the site, causing large-scale material damage and destroying the building," state television quoted the military as saying.
The army also denied reports Israeli forces had launched a strike overnight on a weapons convoy from Syria near the border with Lebanon.
Israel has expressed concern that Syria's stockpile of chemical weapons could fall into the hands of Lebanon's Shiite Hezbollah group, an ally of the Damascus regime, or other militant organisations.
The United States declined to comment on the reported strike by Israel, whose military intelligence chief Aviv Kochavi is currently in Washington for talks with top US general Martin Dempsey.
"I'd refer you to the government of Israel for questions about deliberations or actions that they may or may not have taken," White House spokesman Jay Carney said.
Russia said Thursday it was checking reports of the air strike but would condemn the "unprovoked" attack if the information proved true.
"If this information is confirmed, then we are dealing with unprovoked strikes against targets located on the territory of a sovereign state, which brazenly infringes on the UN Charter and is unacceptable, no matter the motive used for its justification," said a foreign ministry statement.
Russia added that it was taking "urgent measures" to clarify the situation.
"We once again call on the end to all violence in Syria, underscoring the inadmissability of any type of intervention from abroad, and the start of inter-Syrian dialogue based on the Geneva agreements of June 30, 2012," the Russian statement said.
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The reported air strike came after the UN and Arab League envoy to Syria, Lakhdar Brahimi, issued a fresh warning of the conflict spilling over.
"None of the neighbours is immune to the fallout consequences of the conflict," he told the UN Security Council.
International efforts have so far failed to end the conflict, which flared when regime forces launched a bloody crackdown on Arab Spring-inspired protests that erupted in March 2011.
In a surprise move on Wednesday, opposition chief Moaz al-Khatib said on his Facebook page that he was ready for conditional talks with Assad's representatives.
The Syrian National Coalition leader said the conditions included releasing "160,000 detainees" and that embassies abroad renew the passports of exiled citizens.
Until now, Syria's main opposition groups have said they are prepared to talk only 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.
"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."
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, but made it clear this would apply only to groups not linked to the armed insurgency, effectively excluding 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.
Influential opposition group the Syrian National Council -- a key component of the Coalition -- swiftly rejected his 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 lashed out at the "global inaction" it said was giving Assad's regime a licence to kill, blaming his forces for the deaths of at least 78 people whose bodies were found on Tuesday in a river in the northern city of Aleppo.
Violence nationwide on Wednesday killed another 102 people, said the Syrian Observatory for Human Rights, adding to a grim overall death toll that the UN says tops 60,000.