The Swiss authorities have decided to extradite the former Russian atomic-energy minister, Yevgeny Adamov, to the United States rather than to his homeland.
Russia denounced the ruling as politically motivated, adding that the priority for extradition should be given to the country from which a person came.
Adamov, who is facing charges in the US of stealing up to $9 million (SFr11.7 million) intended for improvements to Russia's nuclear security, has 30 days to file an appeal with the Swiss Federal Court.
The justice ministry said in a statement on Monday that the US request was given precedence because "had priority been given to Russia, Adamov's Russian citizenship would have meant that he could not subsequently have been extradited forward" to the US.
This would have resulted in an unacceptable failing of the prosecution process, the ministry said, adding that the US could later deport Adamov to Russia after completing its own judicial proceedings.
The Russian foreign ministry said in a statement: "Over the past few days the Russian foreign ministry has expressed the hope that the decision would be judicial and not politically motivated. Unfortunately this has not happened.
"The (Swiss) Federal justice authorities has taken a decision that runs counter to judicial and objective circumstances," it said.
Russia had been contesting the US request fearing that Adamov could reveal nuclear secrets.
It had also consistently argued that Adamov, as a former government member under ex-president Boris Yeltsin, enjoyed immunity from criminal prosecution by a foreign state for any actions carried out by him in an official capacity.
The justice ministry found that "all the conditions for Adamov's extradition to the US had been fulfilled".
Adamov has accepted extradition only to Russia. He started a hunger strike on Monday, said ministry spokesman Rudolf Wyss, without elaborating on the reason.
Wyss told the AP news agency that the ministry had not been in contact with Adamov since being informed by prison officials of the hunger strike.
But the Russian news agency Interfax quoted Adamov as saying he was staging the hunger strike to press for extradition to Russia or release.
Igor Petrov, spokesman for the Russian embassy in Bern, told swissinfo that the embassy had been informed of the justice ministry's decision. "We are keeping track of events," he added.
Swiss authorities arrested Adamov on a US warrant on May 2, while he was visiting his daughter in Bern.
Grand jury charges
A US federal grand jury in Pittsburgh, Pennsylvania, has since indicted the Russian on charges of conspiracy to transfer stolen money and securities, conspiracy to defraud the US, money laundering and tax evasion.
US authorities suspect Adamov of embezzling US energy department funds and diverting them into private projects in the US, Ukraine and Russia.
In its own extradition demand, filed after the US, Moscow accused Adamov of fraud between 1998 and 2001 during his term of office.
The ex-minister has not denied he put money into private accounts but has said this was normal practice in Russia to shield money from hyperinflation, an unstable banking system and corruption rife after the collapse of communism.
swissinfo with agencies
Yevgeny Adamov was Russia's atomic-energy minister from 1998 to 2001, when a parliamentary commission accused him of corruption.
He is charged by the US with embezzling funds that had been destined for nuclear-safety upgrades in Russia.
Russia had presented a formal extradition request to the Swiss authorities on May 17, accusing Adamov of fraud.
On June 9, the Federal Criminal Court ordered his release, but this decision was overturned by the Federal Court in Lausanne ten days later.