* Kerry, Fabius say more needs to be done to toughen deal * French FM asserts right for Paris to air its views * "Crucial" round of talks to begin March 15 (Adds details, quotes) By Arshad Mohammed and John Irish PARIS, March 7 (Reuters) - The United States and France sought on Saturday to play down any disagreements over nuclear talks with Iran, saying they both agreed the accord **w under discussion needed to be strengthened. "We are on the same page," U.S. Secretary of State John Kerry told reporters after talks with French Foreign Minister Laurent Fabius in Paris. We k**w this." The aim of the negotiations is to persuade Iran to restrain its nuclear programme in exchange for relief from sanctions that have crippled its eco**my. Iran, a major oil exporter, wants the sanctions scrapped swiftly, the powers only in phases.