The United States is expelling a Russian national who is working with the UN Secretariat, said a UN spokesman on Tuesday.

"I can confirm that the U.S. Mission to the United Nations informed the Secretariat on Feb. 28 (of) its decision to take action under Section 13b of the UN-U.S. Headquarters Agreement with respect to a staff member in the Secretariat," said Stephane Dujarric, spokesman for UN Secretary-General Antonio Guterres.

"We regret that we find ourselves in this situation, but are engaging with the host country in line with Section 13b of the agreement," he said.

The spokesman refused to give any more details "in deference to the privacy of the individual concerned and the sensitivity of the matter."

He said the U.S. decision is strange as the work contract for the staff member concerned was scheduled to end on March 14.

The United States on Monday announced the expulsion of 12 staff members of the Russian Permanent Mission to the United Nations.

In a statement, the U.S. Permanent Mission to the United Nations described the 12 Russian diplomats as "intelligence operatives" who have abused their privileges of residency in the United States "by engaging in espionage activities."

Russia's permanent representative to the United Nations, Vassily Nebenzia, on Monday regretted the U.S. decision.

Nebenzia said the expulsion is a hostile step taken by the United States and "yet another gross violation" of the UN-U.S. Headquarters Agreement and of the Vienna Convention.

The U.S. decisions to expel 12 Russian diplomats and one UN staff member from Russia came amid Russian military operations in Ukraine.