Russian Foreign Minister Sergei Lavrov says the Syrian opposition will never be able to defeat President Bashar al-Assad's forces - even if the rebels are armed to the teeth.
Mr Lavrov warned western and Arab nations that supplying weapons to the opposition would lead to carnage, which could last many years.
Saudi Arabia and Qatar have argued in favour of arming the Syrian rebels.
But other countries supporting political change are opposed to the idea, including the United States and Turkey.
In Azerbaijan on Wednesday, Mr Lavrov said the "Friends of Syria" group of Western and Arab nations backing political change were hindering attempts to end the bloodshed.
Gulf Arab states agreed on Sunday to fund the pay of Free Syrian Army fighters. However, critics said the funds could be used to buy weapons on the black market.
A ceasefire is due to begin on 10 April.
