Russian Deputy Foreign Minister Sergei Ryabkov told reporters the eight nations have agreed they should not specify any outcome from peace talks that all agree should start soon in Geneva.
Ryabkov said the talks should aim to create a transitional coalition government for Syria, but should not predetermine whether Assad can participate in that government.
"It would be wrong and harmful and violate the political balance," he said. "We cannot dictate to the participants in the process how it's going to end, otherwise it makes no sense to even start it."
The formal Syria declaration is expected later Tuesday after leaders of all eight G-8 members discussed the Syrian impasse over dinner Monday night.
Ryabkov dismissed Western claims that Assad's forces used chemical weapons as unproven, and said they require further investigation.
"We cannot agree to biased interpretations and groundless claims that the government of Syria has used chemical weapons," he said.
He said a new probe involving an international chemical weapons watchdog and the World Health Organization should be assigned to investigate the validity of such claims.
He said Syria should join an international treaty banning chemical weapons and destroy its arsenal of such weapons.