The US state department has expressed concerns to the Russian government over the harassment of American officials amid reports that two diplomats were slipped date rape drugs in St Petersburg last year.

A man and a woman, both US officials with diplomatic passports, were drugged while they were attending a United Nations anti-corruption conference in November 2015, the news outlet RFE/RL reported, citing unnamed officials. The incident most likely occurred in their hotel bar, investigators concluded.

One diplomat had to be treated at a local hospital before being flown out of the country. Investigators were unable to gather evidence from the hospital or hotel, and Moscow said there was no evidence of drugging when Washington served it a formal note of protest.

When asked about the drugging incident, spokeswoman Elizabeth Trudeau declined to comment on specific cases but said the state department was “troubled by the way our diplomatic and consular staff have been treated over the past two years” in Russia.

“We have raised our concerns at the highest levels,” Trudeau said at a briefing. “In particular, the harassment and surveillance of our diplomatic personnel in Moscow by security personnel and traffic police has increased significantly.”

Western officials and journalists in Moscow have previously reported finding signs that intruders have entered their homes. During Barack Obama’s first term, Russian agents reportedly even broke in and killed the dog of a US defense attache in Moscow.

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But recent incidents suggest Russian security services are increasingly resorting to bizarre and invasive intimidation tactics as tensions with the west rise. Following sanctions against Russia over the 2014 annexation of Crimea, jingoist rhetoric has grown and President Vladimir Putin has warned about “enemies from abroad” and a “fifth column” sowing unrest in Russia.

American officials in Moscow have complained about mysterious nocturnal break-ins during which furniture is rearranged and televisions and lights switched on, the Washington Post reported in June. Officials have returned home to find water faucets left running and, in two cases, human excrement on their floors. US diplomats were also stopped dozens of times by traffic police this spring.

The secretary of state, John Kerry, spoke with Putin about the harassment of diplomats during a March visit. But in June, a Russian federal security service guard stationed outside the US embassy in Moscow attacked a US diplomat who was trying to enter, breaking his shoulder. The Russian foreign ministry said the diplomat was an undercover CIA agent. Moscow and Washington responded with tit-for-tat expulsions of diplomats.

After the June incident, a Russian foreign ministry spokeswoman, Maria Zakharova, said: “The more the US damages relations, the harder it will be for US diplomats to work in Russia.”

• This article was amended on 6 October 2016. An earlier version referred to RFE/RL as a “state news outlet”. To clarify: it is funded by the US Congress but is editorially independent.