The US and Russia meet in Germany Saturday in a new bid to iron out differences over Syria's war, with the leader 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 to meet Russian Foreign Minister Sergei Lavrov, Syrian opposition chief Moaz al-Khatib and UN-Arab League peace envoy Lakhdar Brahimi.
The United States was pushing to help strengthen the opposition, Biden told the conference, insisting Syria's President Bashar al-Assad was a "tyrant" and must go.
Biden said it was "no secret" that Moscow and Washington have "serious differences" on issues like Syria, as fears mount that its 22-month conflict will draw in neighbouring states..
"We can all agree... on the increasingly desperate plight of the Syrian people and the responsibility of the international community to address that plight," he added.
The United States and its allies have made repeated calls for Assad's ouster, while key Damascus ally Russia has resisted any international action.
Lavrov, who would also reportedly meet Khatib, called for another meeting of the Syrian action group led by Brahimi in a bid to find an accord on a transition, saying he believed progress was possible.
On Friday, Brahimi called for a "clear decision" from the UN Security Council to set the agenda for a peaceful solution.
"One has to have some kind of hope, but... I am much more conscious of the difficulties, of the country being broken day after day than I am of a solution," Brahimi said in Munich.
A day earlier, National Coalition chief Khatib 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.
"As a gesture of good will... we are ready to sit at the negotiating table with the regime but we don't want their hands to be full of blood," he said calling for the release of detainees.
Khatib called for "some kind of electronic interference" to stop regime aircraft shelling the Syrian people.
"If that doesn't work I would demand to destroy the planes and weapons of the Syrian regime," he said, nearly two years into a conflict the UN says has killed more than 60,000 people.
Officials in Washington have said Iran is boosting its support for its key ally Assad, and that Moscow is still arming his regime.
Damascus said Israeli air strikes hit a weapons research centre near the city, while intelligence sources said a weapons convoy destined for Lebanon's Hezbollah militia was hit.
The Assad regime threatened to retaliate over the raid and Tehran warned it would have "grave consequences."
Israel has not commented about the reports, but a US official said an Israeli raid struck surface-to-air missiles and a nearby military complex on the capital's outskirts.
"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 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 that Washington is also concerned the "chaos" in Syria could allow Hezbollah to obtain sophisticated weaponry.
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 Syrian Observatory for Human Rights, which relies on activists and medics on the ground for its information, said at least 85 people were killed on Friday -- 32 soldiers, 31 rebels and 22 civilians.