Iranian President Mahmoud Ahmadinejad has recalled a string of ambassadors from high-profile postings.
Mr Ahmadinejad is likely to appoint his own men to key positions
Tehran's senior diplomats in the UK, France, Germany and at the United Nations in Geneva are being replaced.
The ambassadors were closely involved in negotiations over Iran's nuclear programme under former President Mohammed Khatami.
The move is likely to signal a tough new approach to the stalled nuclear issue, correspondents say.
Mr Ahmadinejad came to power in June and has adopted an abrasive approach to foreign policy in recent weeks.
He used a speech to the UN General Assembly to warn foreign nations to stay out of Iran's affairs - seen as a coded message about the country's nuclear programme.
Last week he provoked an international outcry with remarks calling for Israel to be "wiped off the map".
The BBC's Iranian analyst Sadeq Saba says the replacement of these senior moderate diplomats may cause concern in Western capitals, especially at a time the nuclear talks have apparently reached deadlock.
Until recently the UK, France and Germany spearheaded international efforts to cajole Tehran into revealing the extent of its nuclear ambitions.
Mr Adeli was replaced as London envoy after just a year in the job
The US accuses Iran of seeking to develop atomic weapons but Iran insists its nuclear activities are legitimate and are aimed at generating domestic energy.
Negotiations broke down when Iran restarted uranium processing at a key nuclear plant, in defiance of international opinion.
Mr Ahmadinejad's attitude has quickly distinguished him from his more moderate predecessor, Mr Khatami.
Many of the diplomats replaced were closely associated with the former regime.
Mohammad Hossein Adeli, the envoy to London, is a US-educated diplomat who only took up his posting in 2004.
Mr Adeli, who has been accused of corruption and of betraying Iranian national interests by hard-line state newspapers in Tehran, was the first Iranian ambassador to speak fluent English since the Islamic revolution of 1979.
Iran's official IRNA news agency said Mr Adeli had made a request for early retirement, but gave no reason.
According to a report in The Times newspaper in London, up to 20 ambassadors around the world will be replaced in a "purge".
Dr Ali Ansari, an Iranian expert at the University of St Andrews, said the changes could backfire.
"It is typical of the insularity of the regime that it creates this crisis and gets rid of its best diplomats just at a time when things could not be worse on the world stage," he told The Times.