"US citizens residing or travelling in Russia should depart immediately," the US embassy in Moscow said. "Exercise increased caution due to the risk of wrongful detentions."

"Do not travel to Russia," the embassy said.

The United States has repeatedly warned its citizens to leave Russia. The last such public warning was in September after President Vladimir Putin ordered a partial mobilisation.

"Russian security services have arrested US citizens on spurious charges, singled out US citizens in Russia for detention and harassment, denied them fair and transparent treatment, and convicted them in secret trials or without presenting credible evidence," the embassy said.

"Russian authorities arbitrarily enforce local laws against US citizen religious workers and have opened questionable criminal investigations against US citizens engaged in religious activity."

Russia has opened a criminal case against a United States citizen on suspicion of espionage, the Federal Security Service (FSB) said in January.

US prisoners in Russian jails

The most well-known American held in a Russian prisoner was basketball star Brittney Griner.

She was returned to the United States in December in a prisoner exchange with Russian arms dealer Viktor Bout, nearly 10 months after being arrested.

Griner's status as an openly gay Black woman, her prominence in women's basketball and her imprisonment in a country where authorities have been hostile to the LBGTQ community heightened concerns for her and brought tremendous attention to the case.

Meanwhile another American remains in jail in Russia, as he was not part of December's prisoner exchange.

Paul Whelan is a former US Marine who was arrested on suspicion of spying in Moscow in 2018, and sentenced to 16 years in prison.

US President Joe Biden has said his government will "never give up" on trying to secure Whelan's release.