Transition prospects uncertain after Russia insists President Bashar al-Assad be allowed to retain power in some form.
Foreign ministers of world powers are in Geneva to hold talks on developing a common strategy to tackle the bloodshed in Syria, but differences persist between Syrian ally Russia and other countries.
Shortly before the actual conference on Saturday, envoys were trying to overcome one of the main sticking points - Russia's insistence that President Bashar al-Assad be allowed to remain in power in some form, a condition that is a non-starter among Syrian opposition members.
"We haven't reached agreement in advance with Russia and China. That remains very difficult and whether it will be possible, I don't know if this will be possible," British Foreign Secretary William Hague said on arriving for the talks hosted by international envoy Kofi Annan.
Kofi Annan has been hoping for consensus on a proposal involving the formation of a unity government comprising leaders from both the government and the opposition and the likely stepping down of Assad.
Moscow, a long-time ally of the Syrian government and an opponent in principle to what it terms foreign intervention in a domestic matter, has voiced objections to any solution that is imposed on Syria.
Sergei Lavrov, the Russian foreign minister, met Hillary Clinton, his US counterpart, in St Petersburg on Friday. He said there was "a very good chance [of finding] common ground at the conference in Geneva [on Saturday]".
His deputy, Gennady Gatilov, later tweeted Moscow's view of forcing Assad aside: "Our Western partners want to determine themselves the results of the political process in Syria," he said.
"However, this is a matter for the Syrians themselves."
Lavrov, however, said that he had seen some flexibility on Clinton's part during their talks.
"I felt a change in Hillary Clinton's position. There were not ultimatums," he said. "Not a word was said that the document we will discuss in Geneva cannot be touched," he said, a few hours after senior officials in Geneva failed to arrive at a compromise that could be presented to the foreign ministers for approval on Saturday.
A senior US state department official, speaking on condition of anonymity, told the Reuters news agency that "areas of difficulty and difference" remain, but that an accord was still possible.
The foreign ministers of the five permanent members of the UN Security Council - Russia, the United States, China, France and Britain - will attend Saturday's talks along with counterparts from regional powers Turkey, Kuwait, Qatar and Iraq, as well as UN Secretary-General Ban Ki-moon and Nabil Elaraby, the secretary-general of the Arab League.
Annan is seeking backing for a proposal that does not explicitly call for Assad to step down, but does outline the formation of a transition government involving leaders from both sides, and excluding any who would jeopardise stability.
Diplomats told Reuters that Russia had proposed changes to the plan on Thursday, despite initially supporting it, but that the US, Britain and France had rejected the amendments.
Annan said on Friday that he was "optimistic" that the Geneva talks would lead to an acceptable outcome.
Assad on Thursday dismissed the notion of any outside solution to the crisis which has imperilled his family's four decades in power: "We will not accept any non-Syrian, non-national model, whether it comes from big countries or friendly countries.
"No one knows how to solve Syria's problems as well as we do."
The United States and regional powers such as Turkey are under pressure not only from Russia but also members of the rebel movement against Assad.
The opposition Syrian National Council has expressed grave reservations about any transition process that reserves a role for the current president.
Laurent Fabius, the French Foreign Minister, was to hold Paris talks with rebel council chief Abdel Basset Sayda in hopes of persuading him to take a more accommodating line.
The London-based Syrian Observatory for Human Rights said some 4,700 of the more than 15,800 people killed since the uprising broke out since a UN-backed ceasefire brokered by Annan entered force.
On Friday, Syrian government forces used helicopter gunships and tanks to bombard opposition strongholds across the country, including towns in the northern province of Idlib and in the capital Damascus, the opposition said.
The attacks were reported after Mustafa al-Sheikh, a former brigadier general who defected from the Syrian army, said that about 2,500 Syrian troops and 170 tanks had assembled at an infantry school near the village of Musalmieh northeast of the city of Aleppo, just 30km from the Turkish border.
The mass military deployment came after Turkey amassed troops and deployed surface-to-air missile launchers along its southern border with Syria in response to the shooting down of a Turkish warplane by Syrian forces.
A spokesman for the Syrian foreign ministry did not deny that Syrian troops were amassing near the border, but stressed that there were "no hostile intentions from the Syrian side".