"Iran's official announcements on the UN resolution are clearly negative," Fischer, who is on a four-day visit to Iran, told Thursday's edition of the Sueddeutsche Zeitung, according to an advance extract.

But he said that Western diplomats should pursue contacts on all levels in Iran because "radical and reform-minded politicians are having a heated debate behind closed doors" on the matter.

Fischer said he has warned stakeholders in Iran that the country would face serious consequences if it defied the ultimatum the United Nations issued to the Islamic republic on Monday.

The UN Security Council gave Iran until August 31 to halt its controversial nuclear enrichment activities, which the West sees as a bid to acquire nuclear weapons.

"The main thrust of my discussions have been to warn about the negative consequences. I did this without any diplomatic nicety. Iran has to stop uranium enrichment," Fischer said.

Asked if he believed Iran could still be persuaded to do so, Fischer said: "This is the time for clever diplomacy. ... In essence this will depend on the United States' conduct towards Iran. And here there is cause for scepticism."

Hardline Iranian President Mahmoud Ahmadinejad on Tuesday vowed that Iran would not bow to "force and threats", while Iran's parliament speaker Gholam-Ali Hadad-Adel called the UN resolution "worthless".

Fischer has been in Iran since Monday and is due to return to Germany on Thursday.

During his term in office as foreign minister from 1998 to 2005, he was seen as enjoying broad trust in the Middle East. He resigned from parliament in June to take up a teaching post at Princeton University.

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