Russia is concerned that if Syrian President Bashar al-Assad were to leave power immediately, it would trigger a leadership scramble and militants could seize control, 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 organizations," Putin said. "Nobody wants this - but how can it be avoided? After all, they are armed and aggressive."
Putin criticized 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 were 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 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."
He repeated Russia's position that Assad's fate and Syria political future should not be decided by outsiders.
"Should Assad stay, should he go, what forms of political leadership should emerge, what constructs - you and I should not impose these things on the Syrian people from abroad," Putin said.
Russia says it is not trying to prop up Assad but insists his exit must not be a precondition for a peace process. It says it has continued to honor arms contracts with his government.
Moscow has been Assad's most powerful diplomatic ally, blocking Western-backed U.N. Security Council resolutions meant to push him from power or pressure him to end the bloodshed.
Challenged by a reporter who said Assad was using weapons against his own people, Putin said the argument was flawed.
"It is ... not the Syrian people that are fighting against Assad, but militants, well trained and armed," Putin said.
Speaking at a panel with Merkel at the St Petersburg International Economic Forum earlier in the day, Putin said the West should not deliver weapons to rebel forces because they include "terrorist" groups.
"If the United States ... recognizes one of the key Syrian opposition organizations, al-Nusra, as terrorist ... how can one deliver arms to those opposition members? Where will they end up? What role will they play?"
France proposed in May that the United Nations declare the al-Nusra Front a terrorist organization, to differentiate it from other Syrian rebel groups. Washington did so last year and says the group is little more than a front for al Qaeda.
U.S. President Barack Obama decided a week ago to provide military aid to rebels trying to overthrow Assad, citing use of chemical weapons by government forces. Russia says it is unconvinced by the U.S. evidence.