Russia is concerned that a political vacuum will emerge in Syria and militants will seize control if President Bashar al-Assad leaves power now, President Vladimir Putin said on Friday.

"We are concerned about the possible appearance of a political vacuum in Syria if some decisions about a change of government in Syria are taken now," Putin told a joint news conference with German Chancellor Angela Merkel.

"Assad goes today, a political vacuum emerges - who will fill it? Maybe these terrorist organisations," Putin said. "Nobody wants this - but how can it be avoided? After all, they are armed and aggressive."

Putin criticised foreign states that arm Assad's opponents in a more than two-year-old conflict that has killed at least 93,000 people. He defended Russia's weapons supplies to Assad's government, saying they are entirely legal.

Putin said the only answer was an international peace conference that Russia and the United States want to convene.

"There is only one reasonable idea, which everybody supported at the G8," he said, referring to a summit this week in Northern Ireland.

"It is to force all the conflicting sides to come to Geneva...and sit at the negotiating table, stop the violence, and find acceptable forms for the future structure of their state and the provision of security for all ethnic and religious groups."