Brussels: NATO has expelled seven diplomats from Russia's mission to the alliance and blocked the appointment of three others over the nerve agent attack in Britain this month.

NATO Secretary-General Jens Stoltenberg said on Tuesday he had also cut the maximum size of the Russian mission to 20 people from 30, in the latest move by the alliance to reduce what was once one of the biggest national delegations at NATO.

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"It sends a very clear message to Russia that it has costs," Stoltenberg said of the March 4 attack on a former Russian double agent and his daughter, which the West blames on Moscow despite the Kremlin's denials.

Ireland and Moldova were the latest to join more than 20 countries, including Australia, to expel more that 130 Russian diplomats in response ot the nerve agent attack on Sergei Skripal and his daughter Yulia.

Russia denounced the actions as "boorish" and pledged to retaliate.