The US government has complained to the Kremlin after two of its diplomats allegedly had their drinks spiked with date rape drugs in what officials believe is a Cold War-style harassment campaign by Russia.
Two junior US officials, one male and one female, were believed to have been targeted separately while attending the United Nations Convention against Corruption in St Petersburg in November.
The allegations were first reported by Radio Free Europe/Radio Liberty which was briefed by a US government official.
One of the diplomats was incapacitated and taken to a clinic for treatment but US officials believe there was then an attempt to cover up the incident.
They tried to get blood tests done to determine the precise drug that had been used but the electricity went out at the clinic so samples couldn't be taken.
The victim was then flown out of the country to another hospital but by that time it was too late to obtain samples.
When the US officials sought to investigate workers at the luxury St Petersburg where the poisoning allegedly happened they found there were no records of who had been on shift.
According to the official a formal note of protest was lodged with Russia by the US Sate Department.
US State Department spokesperson Elizabeth Kennedy Trudeau, said: "We are troubled, and we remain troubled, by the way our diplomatic and consular staff have been treated over the past two years.
"In particular, the harassment and surveillance of our diplomatic personnel in Moscow by security personnel and traffic police has increased significantly."
Earlier this year video footage emerged of a US diplomat and a Russian police officer in a scuffle outside the US Embassy in Moscow.
Both sides subsequently expelled two diplomats each in a Cold War-style standoff.
It is not unknown for foreign tourists and businessmen to be drugged in Russia, but the location in an upmarket hotel was unusual and there was no attempt to rob either of the US officials.