US President-elect Barack Obama [Images] on Saturday said Iran's development of nuclear weapons was "unacceptable" and sought a concerted effort by the international community to address the controversial issue.

"Iran's development of a nuclear weapon I believe is unacceptable. And we have to mount an international effort to prevent that from happening. Iran's support of terrorist organisations I think is something that has to cease," Obama told a press conference, his first since his victory in the historic November 4 Presidential polls.

Asked whether he had responded to a letter written by Iranian President Mahmoud Ahmadinejad congratulating him on winning the election, Obama said "I am aware that the letter was sent."

 "I will review the letter from President Ahmadinejad, and we will respond appropriately. It's only been three days since the election," he said.

 "Obviously, how we approach and deal with a country like Iran is not something that we should, you know, simply do in a knee-jerk fashion. I think we've got to think it through. But I have to reiterate once again that we only have one President at a time. And I want to be very careful that we are sending the right signals to the world as a whole that I am not the President and I won't be until January 20," he said.

Ahmadinejad's message to Obama marks the first time an Iranian leader has offered congratulations to a US President-elect since the 1979 Islamic Revolution.

Image: Obama at his first press conference after being elected as the US president.

Photograph: AFP/Getty Images