Moscow (Reuters): Russia said that too many American spies operated in Moscow under diplomatic cover and said it might expel some of them to retaliate against the United States over Washington's expulsion of 35 Russian diplomats last year.

The warning, delivered by Russian Foreign Ministry spokeswoman Maria Zakharova, reflects rising frustration in Moscow over the Trump administration's refusal to hand back two Russian diplomatic compounds which were seized at the same time as some of Russia 's diplomats were sent home last year.

Barack Obama, US president at the time, ordered the expulsion of 35 suspected Russian spies in December, along with the seizure of the two diplomatic compounds, over what he said was the hacking of U.S. political groups during the 2016 presidential election, something Russia has flatly denied.

President Vladimir Putin decided not to retaliate immediately at the time, saying he would wait to see what the new administration of Donald Trump would do.

Zakharova complained on Friday that US officials were not issuing visas to Russian diplomats to allow Moscow to replace the expelled employees and get its embassy back up to full strength.

"We have a way of responding," she told a news briefing. "The number of staff at the US embassy in Moscow exceeds the number of our embassy employees in Washington by a big margin. One of our options, apart from a tit-for-tat expulsion of Americans, would be to even out the numbers."

If there was no movement in the US-Russia dispute soon, she said Moscow would have to reluctantly retaliate and suggested US spies working in Russia would be among those to be expelled.

"There are too many employees of the CIA and the Pentagon's espionage unit working under the roof of the American diplomatic mission whose activity does not correspond at all with their status," said Zakharova.