LAUSANNE, Switzerland — Negotiations over Iran’s nuclear program grew frantic on Saturday amid signs of discord, with the French and German foreign ministers joining U.S. Secretary of State John Kerry in talks with Tehran’s top diplomat ahead of an end-of-March deadline for a preliminary deal. With just four days to go until that target, negotiators in the Swiss town of Lausanne were meeting multiple times in various formats to produce what they hope will be an outline of an agreement that can become the basis for a comprehensive deal to be reached by the end of June. Iranian negotiator Majid Takht-e Ravanchi denied a news report that the sides were close to agreement on a text, and other officials spoke of remaining obstacles, including Iranian resistance to limits on research and development and demands for more speedy and broad relief from international sanctions. Iran says its nuclear ambitions are purely peaceful; other nations fear it is seeking to develop weapons. The future of Iran’s uranium enrichment program.