United Nations Secretary-General Ban Ki-moon says Iran must do more to prove its nuclear program is peaceful.
Ban said on February 17 that Tehran had so far been unconvincing in its claims that it has not worked on developing nuclear weapons.
However, he warned against any military action against Iran, saying "there is no alternative" to diplomacy.
Ban was speaking in Vienna, Austria, where he attended ceremonies marking the 15th anniversary of the Comprehensive Nuclear Test Ban Treaty Organization, a UN agency devoted to verifying a planned worldwide ban on nuclear-weapons testing.
A team from the UN's International Atomic Energy Agency is expected to visit Tehran next week to continue an investigation into suspicions that Iran has engaged in nuclear-weapons-related experiments.
Iran has denied carrying out such work.
Compiled from agency reports