MOSCOW (AP) — Syria's warring sides and the mediators meeting in Kazakhstan failed to agree on the formation of a committee meant to draft a new constitution which is key to ending the seven-year civil war, the U.N. special envoy to Syria said Thursday, calling it a "missed opportunity."

Staffan de Mistura was in the Kazakh capital of Astana where Russia, Turkey and Iran are holding talks with the Syrian government and the opposition on steps to bring peace to the country.

The committee had been expected to be formed before the end of the year.

At issue is the 50-member delegation comprising S...