The Iranian nuclear talks between world powers and Iran intensified on Monday with substantial differences still remaining ahead of the Tuesday deadline.For days Iran, the United States, Britain, France, Germany, Russia and China have been seeking to break an impasse in negotiations aimed at stopping Tehran from having the capacity to develop a nuclear bomb in exchange for an easing of international sanctions that are crippling its economy.But officials at the talks in the Swiss city of Lausanne said attempts to reach a framework accord could yet fall apart.A Western diplomat said there are three major sticking points that must be resolved if Iran and major powers are to secure a framework deal before a self-imposed end-March deadline and it is unclear whether those differences will be bridged.The diplomat said the most difficult issues were related to the duration of any limits on Iranian enrichment and research and development activities after an initial 10 years, the lifting of U.N. sanctions and restoring them in the case of non-compliance by Iran."It seems that we have an accord for the first 10 years, but with regard to the Iranians the question of what happens afterwards is complicated," the official said on condition of anonymity, adding: "I can't say what the final result will be."German Foreign Minister Frank-Walter Steinmeier said there had been "some progress and some setbacks in the last hours".Russian Foreign Minister Sergei Lavrov's spokeswoman said he was returning to Moscow later on Monday although officials said he would return to Switzerland if there was something to announce.