Iran has made important strides towards clarifying past nuclear activities but key questions remain unresolved and it has significantly expanded uranium enrichment, a UN watchdog report said on Thursday.

The International Atomic Energy Agency report said Iran had provided much documentation and allowed interviews with nuclear officials involved in developing centrifuges in the 1980s and 90s with designs and parts obtained from the black market.

Centrifuges can refine uranium for power plant fuel or bombs.