berlin u.n. special envoy syria thursday say u.s. president barack obama could well keep work last day office end war syria russia want hold responsible demolition eastern aleppo would never underestimate outgo u.s. president lame duck staffan de mistura say interview publish friday germany sueddeutsche zeitung newspaper president obama .... secretary state john kerry motivate end great humanitarian tragedy century play time office legacy de mistura say de mistura tuesday express concern syrian president bashar al-assad could launch brutal new offensive crush rebel-held eastern aleppo u.s. president-elect donald trump take office jan. 20. say recent meet damascus show syrian regime embolden trump comment campaign end support syrian opposition say remind syrian leaders u.s. president would face massive public pressure situation end human tragedy 200,000 people starve de mistura say fear eastern aleppo could decimate christmas bombardment continue would spur tens thousands refugees flee turkey could lead prolong guerilla war rural areas car bomb cities head syria white helmets volunteer group thursday say besiege residents aleppo ten days starvation u.n. envoy say believe russia claim bomb target eastern aleppo add moscow also stop syrian force attack hospitals target city still say believe russia serious want take responsibility destruction eastern aleppo say simply interest russia syrian government leave completely destroy country five six years mini-afghanistan type guerrilla war rural areas u.n. envoy see continue u.s. work end syria war