Ali Asghar Soltanieh, who has been Iran’s representative to the UN International Atomic Energy Agency (IAEA) since 2005, announced his resignation Thursday.

"My mission is finished... and I return to Iran with satisfaction," he said, without giving a reason for his departure.

He is to be replaced as of September 1, however his successor has not yet been named.

Soltanieh, who holds a doctorate in nuclear science from the Massachusetts Institute of Technology, has played a key role in defending Iran's nuclear program as peaceful and denying allegations that Tehran wants to possess atomic arms. However, during his time as representative there have been fruitless negotiations with the agency.

It was not immediately clear whether his resignation was planned or if it was a more recent decision by Iran’s new President Hasan Rouhani, who was sworn in earlier this month.

Soltanieh’s surprise departure comes less than a week after Rouhani named former Foreign Minister Ali Akbar Salehi as the head of Iran's Atomic Energy Organization. Salehi is considered more pragmatic than his predecessor and the move has been seen as a signal that Rouhani intends to pursue a more flexible approach to Iran's nuclear dispute with the West.

Western countries and Israel suspect Iran's nuclear program is a cover for developing atomic weapons, which Tehran strongly denies.

The IAEA has been investigating Iran’s program for the past decade. Iran has faced several international sanctions for its refusal to stop enriching uranium, a process key to creating a nuclear weapon.

hc/lw (AFP, AP, Reuters)