MOSCOW (AFP) - A high-ranking Russian diplomatic source said on Saturday it was highly unlikely that a proposed international peace conference on Syria could be held this month because of differences over who should take part.

"It is unlikely before the end of May," said the source who participated in talks between Russian President Vladimir Putin and visiting British Prime Minister David Cameron on Friday.

"This is a very complicated matter," he was quoted as saying by Russian news agencies. "It's unrealistic to set some kind of a tight timeframe."

Earlier this week, Russian Foreign Minister Sergei Lavrov and US Secretary of State John Kerry offered to hold a new international conference in search of peace in Syria after 26 months of deadly conflict.

The source said that Mr Putin and Mr Cameron had discussed "a number of preparatory concrete steps on the way to convening some kind of format".

"It is too early to say whether it will be a conference or something else," the diplomatic source was quoted as saying.

"There are too many differences: who can take part in this format, who is legitimate, who is not legitimate.

"It is obvious that this cannot be done without representatives from the opposition but the question is, what opposition," he said, arguing that the opposition in Syria was not united and therefore could not speak with one voice.