Russia has been unimpressed with the American evidence. Instead it is urging the United States and its allies hold off on taking action until a team of UN experts submits its report on the event. Even then, it insists any military action would be illegal unless it is authorized by the U.N. Security Council, something Russia would likely veto. Russia has long opposed Western intervention in Syria and has blocked previous efforts to pressure the Syrian government.
That has led the Obama administration to go around the United Nations. They are trying to build a coalition of countries that would join in the strike. So far only France has said it was willing to participate militarily. Other countries have expressed their support for such a mission but are not willing to commit military resources.
In his remarks to the G20 dinner on Thursday evening, Obama sought to portray his military plan as "limited" in scope and not aimed at toppling the Syrian government. Russia has been skeptical of those claims, pointing to the recent case of Libya where a U.S-led coalition launched "humanitarian" airstrikes that continued until the Gaddafi government was overthrown.