The Group of Eight industrialised nations told Iran on Thursday they wanted a "clear and substantive response" on July 5 to an offer of incentives to stop enriching uranium. But two Iranian officials immediately declared more time was needed.

A Western diplomat familiar with the issue said the Islamic Republic was unlikely to give a firm answer but that if one did not arrive by July 12, when major power foreign ministers next meet, UN Security Council action would loom.

UnderSecretary of State Nicholas Burns insisted the offer was "very straightforward" and Iran's chief negotiator Ali Larijani should respond as requested at a slated July 5 meeting with European Union foreign policy chief Javier Solana.

"There will be a meeting here in this city next week, where we expect and hope that Larijani will give us an answer ... This is not a complicated offer," Burns said in Brussels.

"It is now high time, frankly, that we had a response from the Iranian government ... We always said this was a process of weeks not months," he told a news briefing.

But Russian Defence Minister Sergei Ivanov signaled that a definitive Iranian reply was not expected on July 5.

"At the meeting they will discuss the (offer) and discuss when Iran is ready to distinctively and unequivocally formulate its reply ...," Ivanov told reporters in Moscow.

"It seems unlikely Larijani is planning to come with an unequivocal answer on July 5," the Western diplomat said in Vienna, headquarters of the UN atomic watchdog agency.