Russia has branded Switzerland's decision to extradite former nuclear minister Yevgeny Adamov to the United States as "politically motivated".
The Swiss authorities have denied the charge, saying the ruling announced on Monday was based solely on judicial grounds.
"If the Russians are accusing us, I regret that," said Rudolf Wyss, spokesman for the Swiss justice ministry. "They are disappointed and we are trying to explain the decision to them."
Wyss was responding to earlier claims from the Russian foreign ministry that the decision to extradite Adamov to the US and not to Russia was "at variance with judicial and objective circumstances".
The Russian authorities have been fighting the US extradition request for fear that Adamov could reveal nuclear secrets when he faces charges of stealing up to $9 million (SFr6.9 million) intended for improvements to Russia's nuclear security.
Adamov had earlier agreed to be extradited to Russia, where he faces fraud charges. The former minister has 30 days to appeal against Monday's decision.
"A few days ago Russia expressed the hope that the decision... would be legally, not politically motivated. Regrettably, that did not occur," said the Russian foreign ministry in a statement.
Andre Liebich, central and east European expert at Geneva's Graduate Centre of International Studies, told swissinfo that he thought it was "very likely" that the US had exerted pressure on the Swiss authorities to hand over Adamov.
Prior to Monday's announcement some commentators had suggested that legal conditions favoured Russia since Moscow had handed in its extradition request first and the case involved a Russian citizen.
But the Swiss justice ministry was unequivocal, stating that "all the conditions for Adamov's extradition to the US had been fulfilled".
Liebich said the international tug of war over Adamov was unlikely to improve relations between Washington and Moscow, especially on nuclear issues.
Russia, along with China, is currently opposing plans – backed by the US – to refer Iran to the United Nations Security Council for possible sanctions for violating international nuclear obligations.
Liebich believes the Bush administration has taken a hard line over the affair in order to extract information from Adamov and to "make an example of him".
"Adamov is not Putin's man and as time passes whatever information Adamov has about Russian nuclear programmes is becoming less relevant," he said.
"Still, in Soviet times Moscow would not let anyone who had ever worked on a nuclear programme emigrate – and old habits die hard."
The Swiss justice ministry stressed that the US request had been given precedence because Adamov's Russian citizenship would have meant that he could not have been extradited to the US at a later date.
Wyss said Washington could send him for trial to Moscow once the judicial process was completed and he had served any sentence imposed in the US. "This makes it possible to satisfy the demands of both states," he said.
But Liebich cast doubt on whether Adamov would eventually be extradited to Russia.
"[This will only happen] if US prosecution attempts are frustrated by the courts or if the US government has got whatever information it wants and is willing to let Adamov serve [any future] sentence in Russia. But, in that case, Adamov might not want to return."
The Swiss authorities arrested Adamov on a US warrant in May, while he was visiting his daughter in Bern.
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.
The Americans suspect Adamov of embezzling energy department funds and diverting them into private projects.
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, Adam Beaumont
Former Russian President Yeltsin named Adamov atomic energy minister in 1998.
He kept his post in 2001 after a cabinet reshuffle under President Putin.
However, he was removed from the government later in the same year due to corruption allegations.