Global powers leading negotiations for a nuclear deal with Iran had a "few last" but "tough" issues left to resolve, an Iranian official said Monday.

Three months ago there were a number of unresolved issues, the official said, asking not to be identified. But now "there are only a few items left which need to be tackled by the ministers. That is why the ministers are here," he said, speaking as top diplomats began crunch talks in Vienna chasing a deal.