VIENNA (Reuters) - Senior U.N. nuclear inspectors were expected to return to Vienna early on Friday after two days of talks in Tehran about Iran's disputed atomic activity, a diplomatic source said on Thursday.

There was no immediate sign that the negotiations between the International Atomic Energy Agency and Iran had achieved any breakthrough in unblocking an IAEA investigation into suspected nuclear weapons research in the Islamic state.

(Reporting by Fredrik Dahl; Editing by Mark Heinrich)