Russia's Ambassador to the United Nations Vassily Nebenzia sits at the U.N. Headquarters as the United Nations Security Council assembles to vote for a rare emergency special session of the 193-member U.N. General Assembly on Russia's invasion of Ukraine,
Russia's Ambassador to the United Nations Vassily Nebenzia sits at the U.N. Headquarters as the United Nations Security Council assembles to vote for a rare emergency special session of the 193-member U.N. General Assembly on Russia's invasion of Ukraine, in Manhattan, New York City, U. Reuters / DAVID DEE DELGADO

The United States has expelled 12 Russian diplomats at the United Nations over national security concerns, U.S. and Russian diplomats said on Monday.

The U.S. mission to the United Nations described the Russian diplomats as "intelligence operatives" who had been "engaging in espionage activities that are adverse to our national security."

"This action has been in development for several months," said U.S. mission spokesperson Olivia Dalton.

Russia's U.N. Ambassador Vassily Nebenzia told reporters the diplomats had been asked to leave by March 7. He said Russia would respond to the move "because it's diplomatic practice."

Nebenzia then raised the issue at the start of a U.N. Security Council meeting on the humanitarian situation in Ukraine, describing the U.S. move as "hostile" and a violation of its commitments as host of the U.N. headquarters in New York.

U.S. Deputy U.N. Ambassador Richard Mills responded: "Those diplomats that have been asked to leave the United States were engaged in activities that were not in accordance with their responsibilities and obligations as diplomats."

He said they had been asked to leave "so they do not harm the national security of the host country," adding that the decision was taken in full accordance with the U.N. headquarters agreement.

The United States and its allies have already slapped sweeping economic sanctions on Moscow for its attack on Ukraine - the biggest assault on a European state since World War Two - and moved to ramp up military and other assistance.