At this point in time, there appears to be strong evidence that chemical weapons (possibly a nerve agent) were used. Real questions exist as to who was responsible.
Nevertheless, it appears that the U.S. and some others are inclined to carry out a military operation. The seemingly likely military strikes may be, at least in part and perhaps even the largest part, the result of an emotional response to "do something" on the humanitarian end. General Anthony Zinni spoke about that scenario. From
The Washington Post:
“When there is a humanitarian disaster, people want to see something happen,” Zinni said. “You’ll knee-jerk into the first option, blowing something up, without thinking through what this could lead to.”
Imminent U.S. strike on Syria could draw nation into civil war - The Washington Post
As for Russia's warnings, they likely have to do with unintended and negative consequences that could follow. In the recent wars in Afghanistan and Iraq, the U.S. has not exactly demonstrated a strong capacity for contingency planning, with the result that the U.S. was unprepared for likely insurgencies in both countries. Russia is not in a position to block U.S. military intervention, though it could speed up arms deliveries to Syria and provide Syria with upgraded weapons in the aftermath. It might also conclude weapons deals with Iran to make any U.S. strikes potentially more costly.
The reality is that U.S. military action against Syria, in which Russia has far greater interests than the U.S., is a major problem for Russia. It would further illustrate Russia's inability to protect its allies, much less shape world affairs. It would also exacerbate Russia's worries about its own growing weaknesses.
Russia, including President Putin, seem quite insecure in a changing world. Russia's population is declining, its life expectancy is falling, ethnic unrest persists along parts of its periphery and in some of its Central Asian territory. Although Russia remains a great power (largely military and largely on account of its nuclear arsenal--that latter point is probably the most likely explanation for Russia's current refusal to pursue further reductions in nuclear arms), it is having difficulty reconciling the reality that it is not a superpower with its self-perceptions/ambitions. The West's unilaterally expanding the limited no-fly zone authority in Libya from a civilian protection mission to one of regime change not only eroded Russia's trust in U.S. reliability, but painfully brought home the realities of its impotence to influence such events. I suspect that the combination of its present and historic fears, its weakness, and U.S. policy choices have fueled an increasing tit-for-tat dynamic with the U.S.
Russia's leaders might believe that such a strategy conveys strength. To outsiders, it illustrates weakness. Those decisions have little meaningful impact both on the trajectory of world affairs and the global balance of power. Concrete results, not words or threats, provide foreign policy currency. Failures of deterrence, in this case dissuading the U.S. from launching military action against a Russian ally, undermine such currency.
To be sure, Russia can still create problems for the U.S. beyond Syria. For example, should it suspend cooperation with regard to Iran or even ship more advanced weapons systems to Tehran, that would complicate things for the U.S. Nevertheless, that wouldn't create insurmountable problems for the U.S. The U.S. still possesses the power to prevent a shift in the region's balance of power, to protect vital U.S. interests in the Persian Gulf and regional strategic allies, and to deter Iran from using/sharing nuclear weapons technology should it develop that capacity.
In sum, Russia has much to lose should the U.S. carry out military strikes against Syria. Any outcome other than its dissuading the U.S. from launching military strikes could inflict "losses" on Russia. It can't support those strikes, even if credible evidence revealed the Assad government were responsible for the chemical weapons attack (not yet available), as that would demonstrate that Russia is not a reliable partner to its allies and prospective allies. Thus, it can only oppose military action. However, it is essentially powerless to stop it and that weakness would be exposed should any military action be taken. Such military action would demonstrate Russia's lack of ability to protect is allies from attack.
Therefore, Russia could try to make Syria a litmus test of sorts. It did so with Libya, albeit on a smaller scale, in resolving not to support UN Security Council Resolutions that could authorize the use of force for the foreseeable future. In this case, Russia's interests are greater. Hence, it could try to link policy choices to the outcome in Syria in a bid to create substantive rather than symbolic problems for the U.S. Logical choices would include increased weapons deliveries to Syria, perhaps on a scale to turn the sectarian conflict decidedly in Assad's favor. It could withdraw cooperation with the U.S. in some parts of the world where U.S. and Russian interests are not well-aligned. It could suspend or even with draw from the international nuclear talks with Iran. It could conclude new arms deals with Iran that grant Iran access to some of Russia's most sophisticated defense systems. It likely won't respond with military action of its own, though covert action in Syria can't be ruled out.