Nato is expelling seven Russian diplomats in response to a nerve agent attack in the UK.
The international security organisation's chief said the move would send a message to Russia that there are "costs and consequences" for its behaviour.
Twenty-six countries have expelled Russian envoys in the past two days, in solidarity with the UK.
They all believe Russia was behind the poisoning of two people in Salisbury.
Russian ex-spy Sergei Skripal and his daughter, Yulia, were poisoned on 4 March in the southern English city, and investigators say a military-grade nerve agent was used.
Russian has denied involvement.
Speaking in Brussels, Nato chief Jens Stoltenberg said he would also deny pending accreditation for three Russian staff, and would reduce the size of Russia's mission from 30 to 20.
Nato made a similar move in 2015, in response to the Russian annexation of Crimea. Before that, there were 60 Russia personnel at its Belgium headquarters.
Earlier, Russia accused the US of pressuring other countries to join the mass expulsion of its diplomats.
Foreign Minister Sergei Lavrov accused Washington of "colossal blackmail" and said there were "few independent countries" left in modern Europe.
Mr Skripal and his daughter remain stable but critical in hospital.
Mr Lavrov said it was inevitable that there would be a response to the mass expulsion. He singled out the US for blame.
"When one or two diplomats are being expelled from this or that country, all the while whispering apologies in our ear, we know for sure that this is a result of colossal pressure, colossal blackmail, which unfortunately is Washington's main tool now on the international area," he said.
"It is hard to escape a conclusion that we were right when we stressed several times that there remain few independent countries in the modern world, modern Europe."
Russia's foreign ministry is said to be drawing up a number of possible retaliatory measures for President Vladimir Putin to consider.
