The U.N. nuclear watchdog expressed disappointment on Wednesday over lack of progress during two days of talks in Tehran over Iran's disputed nuclear programme and said its request to visit a military site had not been granted.

In the second such visit in less than a month, a senior team from the U.N. International Atomic Energy Agency (IAEA) had travelled to Tehran to press Iranian officials to start addressing mounting concerns that the country may be seeking to develop atomic arms.

During both the first and second round of discussions, the agency team requested access to the military site at Parchin. Iran did not grant permission for this visit to take place, the Vienna-based IAEA said in a statement after the Feb 20-21 talks in the Iranian capital.

The mission's apparent failure seems likely to add to already soaring tension between the Islamic Republic and Western powers, which have ratcheted up sanctions on the major oil producer in recent months.

It is disappointing that Iran did not accept our request to visit Parchin, IAEA Director General Yukiya Amano said in the statement. We engaged in a constructive spirit, but no agreement was reached.

Iran rejects accusations that its nuclear programme is a covert bid to develop nuclear weapons, saying it is only seeking to produce electricity.

But its refusal to curb sensitive atomic activities which can have both civilian and military purposes, and its track record of nuclear secrecy has drawn increasingly tough U.N. and separate U.S. and European punitive measures.

The IAEA said intensive efforts were made to reach agreement on a document facilitating the clarification of unresolved issues in connection with Iran's nuclear programme, particularly those relating to possible military dimensions.

Unfortunately, agreement was not reached on this document, it said.

(Reporting by Fredrik Dahl)