Iranian workers stand in front of the Bushehr nuclear power plant in Iran in this October 26, 2010 file photo. [Photo/Agencies]

VIENNA - A landmark Iran nuclear agreement was reached Tuesday after clearing final obstacles, and a senior diplomat said it included a compromise between Washington and Tehran that would allow UN inspectors to press for visits to Iranian military sites as part of their monitoring duties.

But access at will to any site would not necessarily be granted and even if so, could be delayed, a condition that critics of the deal are sure to seize on as possibly giving Tehran time to cover any sign of non-compliance with its commitments.

Under the deal, Tehran would have the right to challenge the UN request and an arbitration board composed of Iran and the six world powers that negotiated with it would have to decide on the issue.

Still, such an arrangement would be a notable departure from assertions by top Iranian officials that their country would never allow the UN's International Atomic Energy Agency into such sites. Iran has argued that such visits by the IAEA would be a cover for spying on its military secrets.

On Tuesday morning, the European Union announced that the final plenary of Iran and the six countries negotiating with it will take place at 10:30 Vienna time, followed by a news conference. A Western diplomat said they will endorse the deal, reached in the early morning hours. He demanded anonymity because he was not authorized to comment on the confidential talks.