* Unidentified snipers fire at U.N. convoy
* Russia says U.S. intervention would be a grave mistake
* Evidence has been destroyed, U.S. and its allies say
By Lesley Wroughton and Erika Solomon
WASHINGTON/BEIRUT, Aug 26 (Reuters) - The United States put Syrian President Bashar al-Assad on notice on Monday that it believes he was responsible for using chemical weapons against ...