The North Atlantic Treaty Organization on Tuesday expelled seven Russian diplomats over Moscow’s “lack of constructive response” to the nerve agent attack on a former spy in the United Kingdom earlier this month. The alliance also blocked the appointment of three diplomats to Russia’s mission to NATO.

This follows expulsions of at least 140 Russian diplomats from nearly two dozen countries in the last few days, including over 100 on Monday alone. This is believed to be the largest collective expulsion of Russian diplomats in history, BBC reported.

“The attack in Salisbury was the first use of a nerve agent on NATO territory,” the alliance’s Secretary General Jens Stoltenberg said in a statement. He said the action against the diplomats would send “a clear message to Russia that there are costs and consequences for its unacceptable and dangerous pattern of behaviour”.

“Our actions reflect the serious security concerns expressed by all Allies, and are part of the coordinated international effort to respond to Russia’s behaviour,” the statement read. “They are proportionate, and in line with our legal obligations.”

The maximum size of the Russian mission to NATO is now 20, instead of 30. The Russian mission to NATO is in the same premises as the NATO headquarters in Brussels.

Russia, which has denied any role in the nerve agent attack on Sergei Skripal and his daughter Yulia in southern England’s Salisbury earlier this month, said it would retaliate to the “provocative gesture”. The father and daughter are still in hospital, reports said.