World powers vow to bolster Syria truce

World and regional powers working to end the Syrian conflict vowed to bolster the country's shaky ceasefire Tuesday but failed to set a new date for peace talks. US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov put on their now customary joint appearance, but strains in the peace process were evident. United Nations envoy Staffan de Mistura said he could not invite Bashar al-Assad's regime and the Syrian opposition back to peace talks until there is a "credible" ceasefi