Washington (AFP) - Russia is free to apply to accredit more diplomats to replace the alleged spies that have been expelled from the United States, a State Department official said Friday.

This week, in response to an apparent attempt to assassinate a Russian double agent in Britain with a nerve agent, Washington joined a score of Western allies in expelling more than 150 suspected intelligence officers attached to Russian missions.

In all, 48 alleged Russian operatives working under diplomatic cover for its US mission were on the list -- along with 12 attached to the Russian mission to the United Nations in New York.

Russia's consulate in Seattle will also be closed, but the total size of its diplomatic footprint in the United States might not shrink for long, because the expelled staff could be replaced.

"The United States has expelled 48 Russian intelligence officers, but it is not requiring the Russian bilateral mission to reduce its total number of personnel," a State Department official told AFP.

"The Russian government remains free to request accreditation for vacant positions in its bilateral mission. Any requests for new diplomatic accreditation will be reviewed on a case-by-case basis."

The Russian officers, whom Moscow denies are spies, have until April 2 to leave, although US media reported that Russian planes were already at Dulles airport on Friday to begin their transport.

Russia has responded to the Western moves with matching tit-for-tat expulsions.

But, the US official said, it has not requested that the headcount at Washington's embassy in Moscow be reduced -- as the Kremlin had done when former US president Barack Obama expelled 35 diplomats in 2016.

"We therefore understand that the United States may request new diplomatic personnel to fill the positions of diplomats who have been expelled," he said.

President Donald Trump has often appeared reluctant to criticize his Russian counterpart Vladimir Putin and in a call last week congratulated him on his latest re-election.

But this week's diplomatic expulsions were unprecedented in their scale, and the State Department insists that Washington reserves the right to respond to Moscow's "unjustified" expulsions.