GENEVA — The foreign policy chief of the European Union spent much of Thursday in detailed negotiations with Iranian officials over an agreement to temporarily freeze Tehran’s nuclear program. But the day ended without reports of a breakthrough and with a warning by a leading American lawmaker that he was prepared to introduce legislation next month that would impose new economic sanctions on Iran. The talks on Thursday between Catherine Ashton, the European Union’s top foreign policy official, and Mohammad Javad Zarif, Iran’s foreign minister, were described by a spokesman for Ms. Ashton as “intense” and “substantial.” Their negotiations were to continue Friday morning. Reports in the Iranian news media suggested that the talks might continue into the weekend and could end with the arrival of Secretary of State John Kerry and the foreign ministers of other world powers if a deal appeared close. But whether the remaining issues […]