A top UN nuclear official says his team could "could not find a way forward" in attempts to persuade Iran to talk about suspected secret work on atomic arms.

Herman Nackaerts of the International Atomic Energy Agency says the talks in Tehran were inconclusive, although his mission approached the talks "in a constructive spirit."

Nackaerts spoke to reporters at Vienna airport Wednesday shortly after returning from the Iranian capital.

An IAEA statement published overnight already acknowledged the talks had failed.

Iran denies it has experimented with nuclear arms programs but has refused to cooperate with an IAEA probe on the issue for nearly four years.