At least 21 countries, including the United States and European Union countries, expelled more than 100 Russian diplomats on Monday in a coordinated response against Moscow, whom they accuse of poisoning a former spy in Britain.

This is believed to be the largest collective expulsion of Russian diplomats in history, BBC reported.

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.

The United States alone expelled 60 Russian diplomats, saying, “This attack on our ally, the United Kingdom, put countless innocent lives at risk and resulted in serious injury to three people, including a police officer.” The State Department also called the incident an “outrageous violation of the Chemical Weapons Convention and breach of international law”. The US also said it would close a consulate in Seattle.

The UK earlier expelled 23 diplomats. Besides 16 European Union states, Ukraine has expelled 13 diplomats, Canada four, Norway one, and Albania and Australia two each, CNN reported.

Among the EU countries, Croatian Prime Minister Andrej Plenkovic said he will expel one diplomat, the Czech Republic will expel three, Denmark announced that two diplomats would be expelled, and the Estonian Foreign Ministry said one Russian diplomat and a defence attaché will be sent back. Finland said it will expel one diplomat, while France and Germany announced the expulsion of four diplomats each, who must leave the country within a week.

Hungary said it would expel one diplomat and claimed that he was “also conducting intelligence activities”. Italy will expel two Russian diplomats, Latvia will send back one diplomat and a private citizen who runs the office of a Russian company. Lithuania will send back three diplomats, sanction 21 others and ban 23 more from entering the country. Poland, which will dismiss four diplomats, Netherlands (2), Spain (2), Sweden (1) and Romania (1) also extended their support to the UK.

UK Foreign Secretary Boris Johnson praised the “extraordinary international response” by Britain’s allies.