The US has claimed its diplomats are facing 'significantly increased' levels of surveillance and harassment in Russia.
Yesterday, Radio Free Europe alleged that two US officials with diplomatic passports were drugged in St Petersburg last year while they were attending a conference.
The report suggests US officials have since concluded the incident was "part of a definite pattern".
Elizabeth Trudeau of the US State Department said she was aware of the report but could not comment on specific incidents.
However, she told reporters during a press briefing: "I can say is that we are troubled... we remain troubled by the way our diplomatic and consular staff have been treated over the past two years. We have raised our concerns at the highest levels.
"In particular, the harassment and surveillance of our diplomatic personnel in Moscow by security personnel and traffic police has increased significantly. As we’ve said before, we find this unacceptable."
It comes during a time of high tension between the two countries, with the US yesterday having suspended talks on Syria with Russia.
The White House said the suspension would not affect communications between the two countries on counter-terrorism operations in Syria.