* Al Qaeda seizes key districts of Ramadi, Falluja
* Sunni tribes could be formidable adversary for al Qaeda
* Syria's war spreading sectarianism around region (Adds White House comments)
By Suadad al-Salhy
BAGHDAD, Jan 6 (Reuters) - Al Qaeda gunmen seeking to form a radical Islamic state out of the chaos of Syria's civil war are fighting hard to reconquer the province they once ...