The setting was right, but the atmosphere chilly. After a break of more than five months, talks on reviving the 2015 Iran nuclear deal resumed Monday in the Palais Coburg — the luxury hotel in Vienna where the original pact was signed with much fanfare, in a more optimistic time.

With a more conservative government now in place in Iran, and a new set of Iranian negotiators who have said talks need to start with a complete lifting of sanctions, the mood was somber among Western negotiators. But as the first round of formal discussions ended Monday, they tried to be upbeat.

Enrique Mora of the European Union, who is presiding over the talks, said Iran "recognizes the work done in the past six rounds and the fact that we will be building on that.” But he said that Iran was "insisting on sanctions lifting” immediately, which is likely to be unacceptable to Washington.