Belgium said earlier in the day that as NATO had stripped the diplomats of accreditation last week, it saw no reason for them to remain in the country. Some media reports said NATO's move was connected to a spying case involving an Estonian official.
"We have formally informed the Russian delegation that their accreditation is no longer valid and thus there are no grounds for them to carry out their diplomatic functions in Belgium," Belgian Foreign Ministry spokeswoman Delphine Colard said.
She added that the Russians were to leave the country as soon as possible, stressing however that it was not an issue of expulsion, but rather a loss of accreditation.
"We are waiting for word from the Russian mission as to when they will leave the country and who will come to replace them," she said.
Russia retaliated immediately by announcing it would not send delegates to a Russia-NATO Council meeting later in May.
Russia's ambassador to the military alliance, Dmitry Rogozin, also said Foreign Minister Sergei Lavrov would not attend the Russia-NATO Council session, scheduled for May 18-19.
"Considering... the persistence and stubbornness with which NATO plans to hold military drills in Georgia, and despite all the warnings and taking into account the crude move regarding our officials, the holding of a Russia-NATO Council ministerial session looks inappropriate, at least in the designated timeframe," he said.
He said however that Moscow would not terminate diplomatic relations with NATO over the incident.
The Russian Foreign Ministry earlier said Moscow viewed the expulsion of the two Russian diplomats from NATO's headquarters in Brussels as "a vulgar provocation."
One of the two diplomats in question is the 23-year-old son of Vladimir Chizhov, Russia's EU envoy.
The Russian Foreign Ministry earlier said NATO was attempting to expel the diplomats on "a far-fetched pretext," and urged all NATO member states to consider the consequences of the move
The Financial Times reported last Thursday that NATO's decision was connected to a spy case involving Estonian official Herman Simm, who was convicted in February for passing NATO secrets to Russia.