Russian Foreign Minister Sergey Lavrov says he is returning to the Iran talks, and says prospects for reaching agreement on a framework accord are "good."

Diplomats face a midnight (2200GMT) deadline Tuesday to conclude agreement on the outline of an Iran nuclear deal that would serve as the basis for a final accord by the end of June.

Substantial differences persisted, with officials predicting a long day of talks ahead. Top diplomats of the other four permanent members of the U.N. Security Council and Germany met alone and then with Iran's foreign minister to try to bridge the remaining gaps.

Lavrov said he was returning to Switzerland to rejoin the talks, where he says "prospects for this round of negotiations were not bad, and I would even say good."