Negotiators from Iran, the United States and the European Union have resumed monthslong, indirect talks over Tehran's tattered nuclear deal.

That's even as international inspectors acknowledged Thursday that the Islamic Republic began a new expansion of its uranium enrichment.

The resumption of the Vienna talks, suddenly called only Wednesday, appears that it won't include high-level officials from all the countries initially part of Iran's 2015 deal with world powers.

That comes as Western officials appear increasingly pessimistic of a deal to restore the accord and the EU's top diplomat has warned "the space for additional significant compromises has been exhausted."