The US and Russia meet in Germany Saturday in a new bid to iron out their differences over the war in Syria, with the head of the opposition saying the world must not be a bystander to the tragedy.
On the sidelines of the Munich Security Conference, US Vice President Joe Biden is scheduled to meet Russian Foreign Minister Sergei Lavrov and Syrian opposition chief Moaz al-Khatib.
He is also due to see UN-Arab League envoy to Syria Lakhdar Brahimi, the White House said, amid growing international fears of the 22-month conflict in Syria spilling over its borders and drawing in other regional states.
Officials in Washington have said Iran is boosting its support for its key ally Syrian President Bashar al-Assad, and that Moscow is still arming his regime.
The Damascus regime said Israeli air strikes hit a weapons research centre near the city, while intelligence sources said a weapons convoy destined for the Hezbollah militia in neighbouring Lebanon was hit.
Damascus threatened to retaliate over the reported raid and Tehran warned it would have "grave consequences."
Israel has said nothing about the reports, but a US official said on Friday that an Israeli raid struck surface-to-air missiles and a nearby military complex on the outskirts of Syria's capital.
"There were surface-to-air missiles on vehicles" targeted by the Israeli warplanes, the official told AFP on condition of anonymity, saying they were believed to be Russian-made SA-17 anti-aircraft missiles.
The planes also bombed an adjacent military complex of buildings suspected of housing chemical agents, the official said.
Israel has frequently warned that if Syrian chemical weapons fall into the hands of the Shiite Hezbollah, its arch-foe and close Damascus ally, this would be a casus belli.
US Defence Secretary Leon Panetta told AFP in an interview Washington is also concerned that "chaos" in Syria could allow Hezbollah to obtain sophisticated weaponry.
After Wednesday's alleged Israeli air strike, Damascus affirmed "Syria's right to defend itself and its territory and sovereignty," while Iran's deputy foreign minister warned of "grave consequences for Tel Aviv."
Outgoing US Secretary of State Hillary Clinton said Tehran is stepping up its support for Assad's regime and Russia is still arming it, heightening concerns after Damascus's threat to retaliate over the reported Israeli raid.
"The worst kind of predictions of what could happen, both internally and spilling over the borders of Syria, are certainly within the realm of the possible now," she said on Friday, her last day in office.
"The Iranians have made it clear for some time that keeping Assad in power was one of their highest priorities. We believe they have acted on that by sending in more personnel, not only to help Assad, but to support and advise military security forces."
She also noted Russia was propping up the Damascus regime despite US efforts to work for an international solution to a conflict the UN says has cost more than 60,000 lives.
"We have reason to believe that the Russians continue to supply financial and military assistance in the form of equipment to Assad."
The opposition's Khatib said on Friday said the international community must not be a "bystander" to the "tragedy" of the Syrian people and reiterated his willingness to talk to the regime.
He joined Brahimi for late-night panel talks at the Munich Security Conference on Friday.
"As a gesture of goodwill... we are ready to sit at the negotiating table with the regime but we don't want their hands to be full of blood," Khatib said.
On the ground on Saturday, rebels were reported to have taken control of the Sheikh Said district of Aleppo, Syria's second city, in a strategic victory securing a key route to its international airport.
The Britain-based Syrian Observatory for Human Rights watchdog, which relies on a network of activists and medics on the ground, said at least 80 people were killed in violence nationwide on Friday.