A proposed international peace conference on Syria that aims to bring together President Bashar al-Assad's allies and the opposition will probably not happen until October at the very earliest, a top Russian official said.

Russian Deputy Foreign Minister Gennady Gatilov said additional preparatory talks for the meeting will be held between Moscow and Washington at the end of August, and that the diplomatic schedule was already busy for September.

"It will probably not happen in September because there will be other events," Gatilov told the Interfax news agency late on Monday.

"We are in favour of holding the conference as soon as possible, but we have to take certain realities into account that may have an effect on when this forum is convened."

The so-called Geneva 2 talks were initially agreed in May by US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov at a time when the Syrian rebels were making steady advances on the ground.

Russia, one of Assad's strongest international backers, had initially proposed having the meeting by the end of May.

But as Assad's forces mounted a counter-offensive, the talks were repeatedly postponed because of the opposition's failure on the need or terms under which they would attend.

Additional difficulties were sparked by Russia's insistence that Iran, which has provided the Assad regime with weapons and diplomatic support, also attend the negotiations.

Gatilov said the issue of Iran will be discussed by Russian and US officials when they meet at the end of August "in one of the European capitals that will probably not be Geneva".

Lavrov said after meeting Kerry on Washington on Friday that Russia and the United States were in agreement about the need to stage the talks "as soon as possible", but gave no specific date.

The negotiations are based on the results of a Syria peace conference held in Geneva in June 2012, when world powers agreed on the need to establish a transition government in the war-torn country.

But the sides then failed to agree on whether Assad could play a role in forming the new government, and if his closest representatives could serve on the new interim team.

That failure and the conference's inability to halt the fighting on the ground meant that the peace terms were never inforced.

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