GENEVA—U.S. and Iranian officials said a confidence-building deal on curbing Tehran’s nuclear activities is within reach ahead of a new round of negotiations in Geneva starting Thursday. A senior U.S. official said weeks of detailed discussions between Iran, the U.S. and other world powers have shaped a “first-step” deal that could be forged this week. It would have Iran pause its nuclear program—probably for six months—in return for a suspension of some Western sanctions. The deal would put “additional time on the clock” for the two sides to pursue a comprehensive solution to the decadelong conflict over suspicions that Iran is trying to develop a nuclear bomb, the official said on Wednesday. “For the first time, we aren’t seeing them use this negotiating process simply to buy time,” the official said of Iran. “We are coming to understand each other in terms of what the equation would look like […]