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The main opposition National Coalition, meanwhile, was preparing for a weekend gathering in Istanbul to elect a prime minister and government to run parts of the country "liberated" by rebel fighters.
Initially at odds over Syria, Washington and Moscow on Tuesday sought to find common ground to end the bloodshed in a meeting between US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov.
Russia, the most powerful supporter of Syria's President Bashar al-Assad, this week urged his regime to start a dialogue with the opposition to end the conflict that has cost at least 70,000 lives since it began in March 2011.
After their nearly two-hour meeting in Berlin, Lavrov called on the opposition to "declare itself in favour of dialogue" when it meets various Western leaders, including Kerry, at the crunch talks in Rome on Thursday.
Describing his meeting with Kerry as "constructive," Lavrov said Syrian regime officials had "assured" Moscow that they "have a negotiating team and are ready to start dialogue as soon as possible".
He called on the opposition also to name a negotiating team.
US State Department spokeswoman Victoria Nuland said Kerry -- on an 11-day tour of Europe and the Middle East -- had a "really serious and hard-working session" with Lavrov.
She said most of their discussions on foreign affairs had "focused on Syria and how we can work together to implement the Geneva agreement", which remains hazy on whether Assad needs to step down before talks can begin.
Moscow and Washington have differences over Syria -- Russia is one of the few big powers to maintain ties with Assad's regime and, with China, has vetoed UN Security Council resolutions that threatened sanctions on Damascus.
But the United States has toned down its criticism of Moscow's perceived intransigence over Syria.
"We've been absolutely clear that there needs to be a political transition, and we felt that Russia could play a key role in convincing the regime... that there needs to be that political transition," a State Department official said.
Ahead of the talks, Lavrov had blasted what he termed "extremists" that he said were blocking efforts at dialogue between regime and rebels.
The National Coalition had first said it would boycott the 11-nation meeting in Rome, before changing its mind after the United States and Britain "promised specific aid to alleviate the suffering of our people".
The Washington Post said Wednesday the White House was considering a major policy shift to supply rebels with "non-lethal" aid including body armour, armoured vehicles and perhaps even military training, in a report citing US and European officials.
In Istanbul on Saturday, the National Coalition is to appoint the head of an interim cabinet in a secret ballot, the first of its kind since the outbreak of the uprising.
The names of at least five candidates are already circulating, including Burhan Ghalioun, the former head of the Syrian National Council, one of the main components of the umbrella Coalition.
Others include Osama Kadi, an economist and long-time opponent of the Syrian regime, and former prime minister Riad Hijab, who defected in mid-2012.
On the ground, the combatants in Syria's war seemed deaf to the diplomatic activity, fighting and bombarding each other in the Damascus province towns of Daraya, Zabadani and Zamalka, a watchdog reported on Wednesday.
Fierce clashes raged around the historic Umayyad Mosque in second city Aleppo on Tuesday, as the UN's top political official reported that a record 150,000 people fled the conflict this month.