DW: What could happen in the coming days that has so far not been achieved?

Mark Fitzpatrick: Three sets of key issues remain to be resolved: 1) the timing and extent of sanctions relief; 2) verification, including that of nuclear activities of a possible military dimension (PMD); and 3) limits on research and development of newer generation centrifuges. It is common in a negotiation to save the most difficult issues for the end. So even though these issues have been problematic to date, I do not think it will be impossible to find compromise solutions.

Would it be possible for the International Atomic Energy agency (IAEA) and 5+1 group to give up the request for monitoring Iranian military facilities?

It is absolutely not possible for the IAEA and six powers to forego verification at military facilities. The IAEA has this right at other states that have accepted the Additional Protocol, so it would be impossible to allow Iran to have a more lenient inspection agreement. But the inspection of military sites cannot be unlimited either. The answer is to allow "managed access" as specified in the Model Additional Protocol.

Last week, Supreme Leader Ali Khamenei had specified his "red-lines" for the nuclear negotiations. What is your take on that?

As we have seen in the case of past pronouncements by Khamenei, his supposed "red lines" usually allow for a degree of flexibility or creative interpretation. For example, when he says no "inspections" at military sites, one can draw a distinction between regular IAEA "inspections" and "access" which is more restricted.

Do you see the possibility of Iran working with the IAEA on a basis of trust and honesty?

Verification does not rely on trust and expectations of honesty. Safeguards are designed to discover cheating. Of course, the verification system works best when there is cooperation between the IAEA and the state.

And if the IAEA has reason to remain suspicious, then it will not be able in several years to draw what is called the "broader conclusion" under the Additional Protocol that all nuclear activity in Iran is for peaceful purposes. Unless and until the IAEA can draw this broader conclusion, I don't think concerned nations will want to allow Iran to expand its nuclear program without limit.

How great is the danger that a deal might be struck in Vienna but is killed in Washington or Tehran for domestic political gains?

There remains a danger that opponents of the deal could make trouble in unrelated areas in order to create a bad political climate that would increase distrust and hamper the already complicated processes of implementing a deal. But both Iranian President Hassan Rouhani and the leaders of the six powers want the deal to succeed, so I am hopeful that it will not be undone through political back-stabbing.

Mark Fitzpatrick is a director for Non-Proliferation and Disarmament Programme at the London-based International Institute for Strategic Studies (IISS).

The interview was conducted by Omid Rezakhani.