The UN peace envoy for Syria will host Iran, Russia and Turkey for talks on drafting a new Syrian constitution on September 11-12, a United Nations spokeswoman said Friday.  Syria envoy Staffan de Mistura has been tasked with setting up a committee to write a new constitution for the war-ravaged country.  The main foreign powers backing the project are Syrian regime allies Russia and Iran, as well as Turkey, which supports some opposition groups.  Representatives of the three nations will meet de Mistura over two days at the UNs European headquarters in Geneva, spokeswoman Alessandra Vellucci told reporters.  De Mistura has said he wants to have the constitutional committee in place before world leaders meet at the General Assembly in New York in late September.  That will likely require further talks, including with Syrian President Bashar al-Assads government, but Vellucci said she had no details of additional meetings next month.  De Misturas previous efforts to negotiate an end to the Syrian conflict have achieved no breakthroughs.  More than 350,000 people have been killed and millions displaced since Syrias war started in 2011.