Iran said on Wednesday nuclear talks with six world powers would resume in early September, according to state television, after both sides agreed to continue talking for four more months to try to reach a final agreement on Tehran's nuclear program.

The preliminary accord had been due to expire on Sunday but was extended with some adjustments, after the two sides failed during negotiations in Vienna to meet a self-imposed July 20 deadline for a long-term deal to end the decade-old nuclear standoff.

"The extension of talks shows this is a positive atmosphere," Iranian foreign ministry spokeswoman Marzieh Afkham said during a weekly press conference.

She said the biggest difference between both was in enrichment capacity.

Iran says it is only refining uranium to fuel nuclear power plants or research reactors, not to develop a nuclear weapons capability as the West suspects. The six powers - the United States, France, China, Russia, Germany and Britain - want Iran to reduce its uranium enrichment program significantly to make sure it cannot produce nuclear bombs.

Some other issues that have yet to be agreed upon include the heavy water Arak reactor, sanctions relief and the Fordow enrichment site, said Afkham. Iran wants sanctions on the oil-dependent economy to be lifted as soon as possible.

French Foreign Minister Laurent Fabius said last week that talks would resume in September but gave no further details.