An Iranian negotiator said Friday that expert-level nuclear talks with world powers this week have been "useful," as the two sides pursue a final accord on Tehran's controversial nuclear programme.

"All the involved parties view the sessions as useful," Hamid Baeedinejad, who heads the Iranian delegation of experts, told the official IRNA news agency, adding that the negotiations are "serious and substantive."

Experts from Iran and the so-called P5+1 -- the United States, Britain, France, Russia and China plus Germany -- have been holding closed-door technical talks in Vienna since Wednesday.

Baeedinejad had said earlier that the talks, due to wrap up Friday, would focus on advanced centrifuges and the unfinished Arak heavy water reactor.

Western nations and Israel have long suspected Iran is covertly pursuing a nuclear weapons capability alongside its civilian programme, charges denied by Tehran.

The unfinished Arak reactor is of concern to the West because Tehran could theoretically extract weapons-grade plutonium from its spent fuel if it also builds a reprocessing facility, giving it a second possible route to a nuclear bomb.

A landmark interim deal reached last year requires that Iran freeze or curb its nuclear activities for six months in exchange for some sanctions relief while the two sides try to reach a comprehensive agreement.

Negotiators hope to reach a final accord by July 20, when the interim agreement is due to expire.