The foreign ministers of the United States, France and Germany are reported to have cancelled their travel plans over the next few days.
It's hoped the negotiations, taking place in the Swiss city of Lausanne, will culminate in a document that can be used as a basis for the the comprehensive agreement to be completed by the end of June.
However, officials have warned there's no guarantee they'll get a framework by the end of March, and if they do, a final one in June.
The United States, Britain, France, Germany, Russia and China are seeking a minimum 10-year suspension of Tehran's most sensitive nuclear work. However, Tehran denies charges that its trying to build a nuclear weapon and has repeatedly insisted the programme is for peaceful purpose.
Iran is also is seeking an end to UN-imposed international sanctions which have crippled its economy.