KABUL, Afghanistan — The U.S. peace envoy to Afghanistan said Saturday that for the first time he can report “substantive” progress on all four issues key to a peace agreement in the country’s 17-year war, calling the latest round of talks with the Taliban the “most productive” so far. Zalmay Khalilzad said talks with the Taliban had been exclusively about troop withdrawal and anti-terrorism guarantees. But on Saturday, he said the discussions have broadened to include a timeline for both intra-Afghan negotiations as well as a cease-fire. He declined to give details, however. The talks were to resume Tuesday. Khalilzad said it will ultimately be up to Afghans to decide among themselves the agenda for negotiations as well as the terms of a cease-fire. US envoy hails latest talks with Taliban as the best ever September 1 is the date we are looking for.