There will be no change to Iran's foreign policy after President Mahmoud Ahmadinejad fired the country's foreign minister and replaced him with the atomic chief, officials say.
Manouchehr Mottaki was sacked on Monday while on an official visit to Senegal. No reason was given for the dismissal.
A foreign ministry spokesman says the sacking will have little impact, as policy is "decided at higher levels".
Some Iranian MPs and the media have criticised the unexpected move.
Analysts say it may reflect a power struggle within Iran's ruling conservatives, as Mr Mottaki was seen as a close ally of conservative opponents of President Ahmadinejad in parliament.
The head of Iran's Atomic Energy Organisation, Ali Akbar Salehi, has been appointed to take over the foreign ministry.
"With the change, we will not see any alteration of Iran's basic policies," including nuclear talks with world powers, spokesman Ramin Mehmanparast told reporters in his weekly news briefing at Iran's foreign ministry.
Mr Mehmanparast declined to explain why the dismissal took place while the minister was in the middle of an overseas visit, but said Mr Mottaki was due back in Iran later on Tuesday.
Several Tehran newspapers expressed shock and surprise at the abrupt dismissal.
The hardline daily, Kayhan, which is close to Supreme Leader Ali Khamenei, called it a "clear insult" to the veteran diplomat, who had been foreign minister since 2005.
The paper suggested that the firing was the result of a dispute between Mr Ahmadinejad and Mr Mottaki over "parallel diplomacy", which flared up in summer after the president tried to name his own aides as special envoys to work alongside the foreign service.
Mr Ahmadinejad only backed down after Ayatollah Khamenei intervened.
Lawmakers, who will have to approve any new ministerial appointment, also criticised the move and the way it was carried out.
"Of course, I do not approve of the manner Mr Mottaki was dismissed, because he was on duty in Senegal when his dismissal order was issued," Esmail Kowsari, a senior member of the National Security and Foreign Policy Commission was quoted as saying by Fars news agency.
The commission would review the appointment of Mr Salehi as the interim foreign minister, he added.
Mr Salehi, 61, was appointed Iran's atomic energy chief in July 2009. He has been a driving force behind the country's nuclear programme.
But neither Mr Salehi nor his predecessor are on the Iranian negotiating team that is in talks with world powers on Iran's controversial programme, which the West suspects is aimed at acquiring nuclear weapons. Tehran insists it is for civilian energy purposes.
Further talks are scheduled for next month in Iran's neighbour, Turkey, between Iran and the group of P5+1 - the UK, China, France, Germany, Russia and the US.