Sergei Magnitsky: Russia angry over US visa ban
Russia and the US have become embroiled in a fresh row over the high-profile prison death of Russian lawyer Sergei Magnitsky two years ago.
The Russian foreign ministry says the US decision to place a visa ban on Russian officials linked to the death could harm diplomatic relations.
Magnitsky, who was detained after accusing officials of fraud, reportedly died due to torture and neglect.
His case, which sparked outrage, remains under investigation in Russia.
On Wednesday, Washington announced that it was placing an unknown number of officials - whom it did not identify - on a US visa blacklist because of their alleged involvement in the death.
"We do issue visa restrictions on individuals around the world. In this specific case it was the individuals we believe are responsible for his death," said Mark Toner, a US state department spokesman.
The state department said it was part of a broader initiative to target human rights violators.
"The Magnitsky case has long been an issue of concern between us and Russia and we've raised it with them many times," Mr Toner said.
The move has sparked anger in Moscow, with Russia's foreign ministry warning that the move could strain diplomatic relations between the two nations - which had been improving - and saying they would retaliate.
"I can confirm that the Russian foreign ministry, on the order of the president (Dmitry Medvedev) is working on measures against US citizens which are the same as those announced by the state department," Kremlin spokeswoman Natalya Timakova said.
'Protecting the powerful'
Earlier this month, Russian prosecutors said they had opened a criminal investigation into two prison officials.
The officials were named Larisa Litvinova, chief physician at Butyrskaya prison, and the prison's deputy chief Dmitry Kratov - who they said were suspected of negligence causing death.
However the officials were not the same as those named by President Dmitry Medvedev's human rights council, which had produced a report only two weeks before.
The report had concluded that there was reasonable suspicion that Magnitsky's death was triggered by beatings while in police custody, and singled out senior interior ministry investigator Oleg Silchenko and prison chief Ivan Prokopenko as being at fault.
Magnitsky's former employer claimed earlier this month that Russia was "trying to create the appearance" of acting, but was actually protecting the more powerful people involved.
The company, equity fund Hermitage Capital, was accused by Russian officials of evading paying taxes.
Hermitage boss William Browder accuses Russian officials of using his company to operate a $230m (£143m) tax fraud.
Magnitsky, the firm's lawyer, claimed to have unearthed evidence that implicated the police, officials and bankers in the fraud.
He was later arrested, himself accused of fraud, and investigated by some of the very same people he had accused of corruption.