geneva unite nations say thursday extend round syria talk geneva dec. 15 aim shape political solution end war presidency yet discuss u.n. mediator staffan de mistura say talk would focus particular new constitution u.n.-supervised parliamentary presidential elections well 12 core principles decline enumerate discuss issue presidency discuss 12 principles see broad nature impact everything future constitution tell news conference essential refer could share vision kind syria syrians want live say syria opposition always say president bashar al-assad must step negotiators refuse discuss issue recent successes battlefield strengthen hand want believe issue come syrians u.n.-supervised elections de mistura say two weeks ahead round effectively begin say note government negotiators arrive late might take days consult refresh damascus return geneva around tuesday thursday begin shuttle two side instal separate room corridor say contact person less important meet substance atmosphere professional serious side say talk solid diplomatic back recent support russian president vladimir putin u.s. president donald trump assad well telephone call u.s. secretary state rex tillerson start round defeat islamic state main strongholds syria also produce moment truth take place quite backdrop normal round talk de mistura say see people talk involve conflict first time take position direction political dialogue first time eight round syria talk preside de mistura opposition represent unify negotiate team raise possibility direct talk two side u.n. extend syria round dec. 15 presidency yet table