Kerry's spokeswoman said on Sunday he regretted missing the dedication ceremony for an institute named after the US senator, who was a mentor to him.
Officials close to the talks said the French and German foreign ministers, Laurent Fabius and Frank-Walter Steinmeier, were considering delaying a planned joint trip to Kazakhstan in order to focus on clearing the remaining hurdles to a deal.
Iran denies charges from the West and Israel that it wants to build a nuclear weapon, and says its programme is purely peaceful. It wants the removal of international sanctions that are hurting its economy.