Iran's ambassador to the UK has returned to London, four days after being temporarily recalled to Tehran amid a growing row between the countries.
Iran had said it would not recall its ambassador
Morteza Sarmadi left for Iran on 3 September following diplomatic wrangling with the UK Government over a former Iranian diplomat British police are holding in custody.
Hade Soleimanpour was arrested in Durham on an extradition warrant from Argentina - which alleges he was involved in a 1994 bombing that killed 85 people in Buenos Aires.
Iran says the temporary recall of its ambassador was made in protest of the UK Government's refusal to intervene in the legal process following Mr Soleimanpour's arrest.
"We think the British Government has learned that the detention is unacceptable for Iran," Foreign Ministry
spokesman, Hamid Reza Asefi, said.
"Diplomatic channels are still open to find a way out of the problem through unconditional release of
The BBC's diplomatic correspondent, Barnaby Mason, says that while the UK Government is insisting that the extradition process is a matter for the courts and it cannot interfere, this is not entirely true.
Mr Soleimanpour's treatment (R) has strained ties
In reality, extradition under British law is not a purely judicial affair.
The final decision is made by the home secretary, but he gets involved only after the courts have given their ruling, our correspondent says.
At that point, the home secretary may consider political and other representations - for example, from the Foreign Office and the Iranian Government.
Mr Soleimanpour, who has been working as a research assistant at Durham University, has been ordered by the courts to remain in custody until his extradition case begins on 19 September.
Argentine authorities want to question him in connection with the bombing of a Jewish community centre in 1994.
In total, Argentina is seeking the extradition of eight Iranian officials from various countries in connection with the bombing.
Iran previously threatened to recall diplomats from its UK embassy, but had said these would not include its ambassador.
It's officials say the arrest is "politically motivated."
Iran has also suspended economic and cultural ties with Argentina in protest against the arrest.
The British embassy in Tehran remains closed after a shooting incident on 3 September.
Tehran newspapers reported an unidentified person or persons fired five shots at the British embassy breaking windows.