The U.S. has expelled 12 Russian diplomats for allegedly "engaging in espionage activities" that threaten national security, a spokesperson for the U.S. Mission to the United Nations said Monday.

The 12 "operatives" abused their privileges of U.S. residency, said spokesperson Olivia Dalton. The move to expel the diplomats "has been in the works for several months," Dalton said.

Russia's ambassador to the United Nations, Vassily Nebenzia, told reporters that diplomats were asked to leave by March 7. Nebenzia called it "another hostile" measure against the Russian Mission to the United Nations after receiving the intel during a news conference.

Nebenzia told reporters the expulsion "grossly violated" the host country's commitment to the United Nations.

Dalton said the move was in line with the headquarters agreement.

The U.S. Department of State said no further information was available.

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