U.S. Secretary of State John Kerry has said the window for diplomacy over Iran's nuclear program is "cracking open" ahead of new negotiations with between Iran and world powers.
The Islamic Republic is set to hold talks Tuesday and Wednesday with the five permanent U.N. Security Council members and Germany.
Kerry's comments came even as a senior Iranian negotiator said Sunday that Tehran will not transfer its nuclear material out of the country.
Deputy Foreign Minister Abbas Araghchi told Iranian state television that the removal of nuclear material from Iran is a "red line."
In previous rounds of negotiations, world powers called for Iran to give up its existing stockpile of uranium enriched to 20 percent purity and send it abroad. Uranium of that purity is a short technical step away from being converted to weapons-grade material.
Araghchi said that Tehran, however, "will negotiate" about its "level" of uranium enrichment.
The nuclear talks will be the first since Iranian President Hassan Rouhani took office in August, promising to lead a diplomatic effort to ease Western sanctions on his country.
His predecessor, Mahmoud Ahmadinejad, defied U.N. Security Council demands to suspend Iran's uranium enrichment program, which Western powers believe is aimed at making nuclear weapons. Iran insists its nuclear program is for peaceful purposes.