Negotiations over Iran's nuclear program are entering a critical phase with differences still remaining just two days before a deadline for the outline of an agreement.

With the March 31 target fast approaching, the top diplomats from the five permanent members of the U.N. Security Council, Germany and Iran were meeting Monday to try to bridge the remaining gaps and hammer out the framework deal that will be the basis for a final accord to be reached by the end of June.

U.S. Secretary of State John Kerry and his Iranian counterpart have been meeting in Switzerland since Thursday in an intense effort to reach a political understanding on terms that would curb Iran's nuclear activities in exchange for sanctions relief.