Iran has again rejected a deadline to respond to an international offer aimed at resolving a nuclear standoff, saying it would answer during the next Iranian month which begins July 23.

"A deadline is not an issue. We think such statements are not constructive and they will not help resolving the problem. We will respond next month," foreign ministry spokesman Hamid Reza Asefi told reporters.

World powers on Thursday gave Iran one more week to provide a "clear and substantive response" to an international proposal over the crisis.

The five permanent UN Security Council members -- Britain, China, France, Russia and the United States -- plus Germany have offered Iran a package of incentives that includes multilateral talks if it agrees to temporarily halt uranium enrichment.

That work is at the centre of fears the hardline regime could acquire nuclear weapons, although Tehran insists the project aims only to provide fuel for nuclear energy.

Rejecting allegations that Iran was seeking to buy time, Asefi said: "it is not a question of tactics and wasting time. It is a multi-dimensional package and takes time to examine".

"There are ambiguities (in the package) which need to be discussed with the Europeans".

He said several committees were studying the offer and that Iran would deliver its response "sometime" after July 23.

"We will submit a logical response considering our country's rights and interests," he said, referring to the demands for a freeze in enrichment, a process which provides fuel for nuclear plants but can also form the core of an atomic bomb.

"Alleviating the West's concerns should not be interpreted as sacrificing our interests," Asefi added.

Iran's top nuclear negotiator Ali Larijani and the EU foreign policy chief Javier Solana are to meet Wednesday on the international package.

"The two sides will talk about their views on removing ambiguities and reaching a result," Asefi said.