washington unite state friday urge supporters syrian government press participate fully negotiations opposition say lack political resolution war-torn country threaten indefinite instability statement spokeswoman state department say unite state want government supporters use leverage urge regime participate fully tangible negotiations opposition geneva unite state urge party work seriously toward political resolution conflict face continue isolation instability indefinitely syria spokeswoman heather nauert say statement unite nations negotiator characterize round peace talk end thursday miss opportunity lay blame feet syria government u.s. say want syrian government negotiate 'seriously opposition