The United States has taken aim at Russians working at the United Nations, leveling accusations of espionage for the second day running on Tuesday, in a Cold War-like twist as tensions soar over Moscow's invasion of Ukraine.

The American mission to the UN told AFP Tuesday that Washington asked the United Nations to expel an employee allegedly working as a "Russian intelligence operative."

Coming the day after the United States said it is expelling 12 diplomats from Russia's UN mission for "espionage," the request plunged the global body's headquarters on New York's East River -- a place already described by diplomats as a "nest of spies" -- into a climate of deep suspicion.

The United States "initiated the process to require the departure of one Russian intelligence operative working at the United Nations who has abused their privileges of residence in the United States," a US spokesperson said.

When asked if there was any link with Russian President Vladimir Putin's invasion of Ukraine, the spokesperson said that the request, like the expulsion of the 12 diplomats, had been in the works for "months."

UN spokesman Stephane Dujarric confirmed the request and said the employee was a "staff member in the secretariat" -- but would not indicate whether they worked in UN Secretary-General Antonio Guterres' Manhattan offices.

The employee's contract expired on March 14, he added -- making the request "a little difficult to understand."

But "in deference to the privacy of the individual concerned and the sensitivity of the matter, I will not comment further," Dujarric said.

The Russian mission said it was a matter between the UN and the employee in question.

Russia's ambassador to the UN, Vassily Nebenzia, briefly exits a Security Council meeting that was addressing Russia's invasion of Ukraine at the United Nations headquarters on February 28, 2022 in New York City
Russia's ambassador to the UN, Vassily Nebenzia, briefly exits a Security Council meeting that was addressing Russia's invasion of Ukraine at the United Nations headquarters on February 28, 2022 in New York City GETTY IMAGES NORTH AMERICA via AFP / Michael M. Santiago

Washington's order for the Russian UN diplomats to leave by March 7 prompted fury from Moscow on Monday.

A spokeswoman for the US mission to the UN said those told to leave had "abused their privileges of residency in the United States by engaging in espionage activities that are adverse to our national security."

"This is a hostile move against our country," Russian Ambassador to the United States Anatoli Antonov said in a statement posted to Facebook, adding that Moscow was "deeply disappointed" and "totally rejected" the US claims.

The deputy head of the Russian mission, Dmitry Polyanskiy, told AFP the expulsion order did not include either himself, the Russian ambassador to the UN Vassily Nebenzia or the Russian deputy ambassador for African Affairs, Anna Evstigneeva.

Russian diplomats have said the US actions are in violation of its commitments as a host country of the United Nations to allow diplomatic staff to live and work there.

Washington insists however that it has respected the rules.

The Russian mission to the UN has around 100 staff, a Russian diplomatic source said.

Both the announcement of the expulsion and the decision to request the departure of a Russian UN employee came on the last day of Russia's presidency of the UN Security Council, which rotates monthly.

On Tuesday, the United Arab Emirates took over the chair for the month of March.