UN Secretary General Ban Ki-moon on Friday said it was up to Iran to show that its nuclear programme is peaceful, DPA reported.
"The onus is on the Iranian side to convince the international community that their nuclear development programme is genuinely for peaceful purposes," he said.
"To my mind, and to the mind of the IAEA, they have not been able to convince the international community," he told reporters in Vienna, adding that he was "deeply concerned" about indications that the Islamic state was working to develop a nuclear weapon.
In Washington, European Union foreign policy chief Catherine Ashton said she was both "cautious and optimistic" about an Iranian letter indicating its willingness to return to talks about its nuclear programme.
After meeting with US Secretary of State Hillary Rodham Clinton, Ashton said the letter received earlier this week was being evaluated to see if it was substantive.
Clinton and Ashton discussed the letter during meetings at the US State Department and were also in close consultations with other members of the six world powers that have negotiated with Iran on its nuclear programme in the past.
"This response from the Iranian government is one we've been waiting for and if we do proceed it will have to be a sustained effort that can produce results," Clinton said.
Clinton said the process of evaluating the letter among the diplomatic group of the United States, Britain, France, Germany, Russia and China, would take time, but that Iran appeared to have acknowledged the importance of discussing its nuclear efforts without preconditions.
"We think that this is an important step and we welcome the letter," she said.
Meanwhile, senior officials of the International Atomic Energy Agency (IAEA), the UN nuclear watchdog, were to travel to Iran on Sunday to seek answers about the country's nuclear weapons programme.
A diplomat close to the IAEA said the agency "would like to see a commitment from the Iranian leadership" that it is really willing to give the nuclear watchdog access to documents, officials and locations.
IAEA chief Yukiya Amano reported late last year that there were numerous indications that Iran was carrying out projects geared to develop nuclear weapons. Tehran says it needs nuclear technology only for generating power and for other civilian purposes.
"Now he has to give Iran the opportunity to react positively to this report," the diplomat said, indicating that the upcoming trip was not expected to yield a breakthrough.