I don't doubt that Kerry went to Moscow to get Putin to agree that the USA could do whatever it took to unseat Assad. And I also don't doubt that Putin just read him the riot act. After all, there is no way that the USA would agree that sharing the running of an "international conference" with anyone is A Jolly Good Idea, Why Didn't We Think Of It Before?
13 security personnel killed in Iraq suicide bombing (DPA)
from the article: Russia, U.S. call for Syria conference with both sides