Iran's top security official says the "bitter experience" with the United States and “inaction” from the Europeans has made it hard for Iran to trust the other side in Vienna talks.

The secretary of Iran’s Supreme National Security Council, Ali Shamkhani, made the remarks in a meeting with a number of the members of the parliament on the latest developments in the negotiations to revive the 2015 nuclear deal on Wednesday.
He stressed the need to speed up the process for resolution of the remaining issues saying, "Bitter experience with the United States and Europeans inaction have made it inevitable [for Iran] to require guarantees for a reliable, balanced and sustainable agreement."
Shamkhani added that “the main strategy of the country, along with seeking to lift sanctions, is to focus on creating mechanisms to neutralize the instruments of sanctions, and fortunately, very good measures have been taken in this regard”.
The Islamic Republic keeps saying that nuclear talks have made significant progress, but very tough issues remain, apparently insisting it wants an iron-clad guarantee by the United States that it would never renege on a new agreement.
'The bitter experience' refers to the former president Donald Trump’s decision in 2018 to withdraw from the JCPOA. However, the Biden Administration in response has said it cannot constitutionally provide such a guarantee if the agreement is not a treaty ratified by the Senate.