Syria's opposition on Wednesday made a fresh call for a transition to democracy without President Bashar al-Assad, as France threw its support behind new Russia-US talks aimed at ending the war.
The opposition's proposal during one-day talks in London came as the world's chemical weapons watchdog said it was "disturbed" by the alleged use of toxic chemicals in the battleground city of Aleppo.
Moscow said Russian Foreign Minister Sergei Lavrov and his US counterpart John Kerry, whose countries back opposing sides in the brutal civil war, would meet Thursday and Friday in Geneva to try to reach a deal on cooperation in Syria.
The pair have agreed to a "personal meeting" to work out an accord on fighting jihadists and pushing the peace process forward, according to the Russian foreign ministry.
But Kerry's spokesman would not confirm the meeting, suggesting last-minute negotiations were ongoing.
The talks come after US President Barack Obama and his Russian counterpart Vladimir Putin failed to reach a deal to stem the violence on the sidelines of a G20 meeting in China.
French Foreign Minister Jean-Marc Ayrault, attending the London talks, said he backed new US-Russia discussions.
"We are supporting the process, although we want a genuine ceasefire," he said.
With prospects for peace elusive, the broad-based opposition High Negotiations Committee (HNC) -- holding talks with its foreign allies in London -- proposed a six-month negotiating phase with the regime.
The opposition hopes new talks would result in an 18-month transition that would see Syria governed by a transitional body made up of opposition figures, current government representatives and members of civil society, according to a 25-page blueprint.
The HNC and its predecessors -- weakened by years of conflict, regime advances, the spread of jihadists and what some have branded a disconnect from the grim reality on the ground -- have made similar calls since early in the war.
Its proposal came even as armed rebels faced enormous pressure on the ground, particularly around Aleppo where regime forces backed by the Russian air force have completely encircled opposition-held neighbourhoods.
The group's plans are largely in line with existing international proposals for a post-war Syria, although unlike a 2015 agreement hashed out in Vienna, they are clear about Assad's future.
- 'No role' for Assad -
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A transition, HNC says, "shall require the departure of Bashar al-Assad and his clique who committed heinous crimes against the Syrian people."
HNC leader Riad Hijab, speaking in London ahead of the talks, insisted that regime leaders who killed Syrians "cannot have a role in the future of Syria and in this transitional phase at all".
Hijab, a former Syrian prime minister who defected to the opposition in 2012, blamed the failure of previous peace talks on "a refusal to talk about the political transition".
The proposal came as the conflict raged relentlessly and on multiple fronts.
On Wednesday the world's chemical weapons watchdog condemned the alleged use of toxic substances in Aleppo, once Syria's economic capital, after dozens of people had to be treated for breathing problems.
"Such allegations are taken very seriously. The use of chemical weapons by anyone, anywhere and under any circumstances is unacceptable," Ahmet Uzumcu, head of the Organisation for the Prohibition of Chemical Weapons, said in a statement.
More than 70 people were left choking Tuesday after regime helicopters dropped barrel bombs on a rebel-held district of Aleppo, according to a monitoring group.
France's Ayrault said the UN Security Council should condemn the Assad regime and IS "for their continued use of chemical weapons".
- 'Barbaric military tactics' -
Writing in The Times newspaper, British Foreign Secretary Boris Johnson, the host of Wednesday's London talks, urged Russia to cease its support for the Syrian president.
Johnson accused Assad of "barbaric military tactics", blasting Russia's "seemingly indefensible conduct" in backing him.
"The entire international community is committed, at least in principle, to getting rid of the Syrian dictator. Even the Russians have accepted that there must be political transition," he wrote.
"But then the Russians are also employing their military muscle to prevent him from losing and to keep him in power."
The talks in London also involved foreign ministers from Turkey, Jordan, Qatar, United Arab Emirates, Italy, as well as representatives of Germany and the EU. Kerry took part via video-link.
The Syrian war, which began as a pro-democracy revolt in 2011 but morphed into a multi-front conflict after the regime unleashed a crackdown, has killed more than 290,000 people and forced more than half the population to flee their homes.