Russia said Thursday it backed a political transition in Syria but rejected Western pressure to call for the exit of President Bashar al-Assad, ahead of international talks on ending the spiralling conflict.

Foreign Minister Sergei Lavrov poured cold water on hopes that world powers have already agreed a common strategy ahead of the weekend talks in Geneva, saying Moscow was opposed to any solution imposed from abroad.

He also said it was a mistake to exclude Syria's ally Iran from Saturday's meeting of world and regional powers.

Assad's fate "must be decided within the framework of a Syrian dialogue by the Syrian people themselves," Lavrov told a news conference with his Tunisian counterpart.

"Foreign players should not be dictating their solutions to the Syrians. We do not and cannot support any intervention or solutions dictated from abroad," he said.

Lavrov said there was "clearly" a need for a political transition in Syria but said world powers had still not agreed on a final document based on proposals by UN-Arab League envoy Kofi Annan for the talks.

"There are no agreed drafts. Work on a possible final document continues," he said.

Lavrov complained bitterly that "individual working formulas" had been leaked to the press overnight. "I view this as a manifestation of an unfair approach to diplomacy."

Diplomats at the United Nations in New York said Annan has proposed setting up a Syrian transitional government in a bid to end the country's near 16-month conflict and that world powers generally back the plan.

Lavrov agreed that changes and reforms were much needed in Syria, saying: "We support changes which work towards national agreement on all questions of overdue reform".

He said it was a mistake to exclude Syrian ally Iran from the Geneva talks, accusing the United States of "double standards" in opposing Tehran's attendance.

"Iran is an influential player in this situation and to leave it out of the Geneva meeting, I believe, is a mistake," Lavrov said, noting that Washington had agreed in the past to Iran joining talks on Iraq and Afghanistan.

"When the Americans needed to decide certain issues involving the security of their contingents in Iraq and Afghanistan, they initiated contact with Iran without wavering and agreed to something," Lavrov said.

US officials have conceded Iran's influence over Syria while arguing that it had shown no good will thus far that could help support a political transition period backed by Western powers and Turkey.