Russia will expel UK diplomats “soon”, its foreign minister has said, in response to the expulsion of 23 of its diplomats by Britain.
Sergei Lavrov told Russian media the expulsions would “definitely” happen.
The UK took its action after Moscow refused to explain how a Russian-made nerve agent was used on a former spy and his daughter in the UK.
Russia denies it was involved. Its foreign ministry called the allegations by Prime Minister Theresa May “insane”.
Russia described the UK’s position as irresponsible and not backed up by evidence.
When asked when the expulsions would happen, Mr Lavrov was quoted as saying: “Soon. I promise you that.”
He reportedly referred to the UK allegations against Russia as “absolutely boorish”.
Mr Lavrov also said the UK’s approach to the matter was partly prompted by the government’s problems over Brexit.
Former spy Sergei Skripal, 66, and his daughter, Yulia Skripal, 33, remain critically ill in hospital after being found slumped on a bench in Salisbury, Wiltshire, on 4 March.
The chemical used in the attack has been identified part of a group of nerve agents developed by Russia known as Novichok, Mrs May said.
The prime minister said there was “no alternative conclusion” than to believe Russia was “culpable” for the poisonings.
She had given Russia a deadline of last Tuesday evening to explain the use of the nerve agent.
Mr Lavrov then said Moscow would co-operate if the UK submitted a formal request under the Chemical Weapons Convention, which gives a 10-day time limit to respond.
But after Tuesday’s deadline passed, the UK announced the expulsions.
Foreign Secretary Boris Johnson stepped up the war of words with Russia on Thursday.
He said the attack was Russia’s “way of saying to people this is what happens to people who stand up to our regime”.
Mr Johnson said: “Now is the moment for [President Vladimir] Putin to jam the lid down and send a signal to people: ‘You do this, you’re going to die.'”
He told the BBC a sample of the nerve agent would be sent to the Organisation for the Prohibition of Chemical Weapons – the independent international body set up to stop chemical warfare – for analysis.
But he added that Russia’s “smug, sarcastic” response indicated their “fundamental guilt”.
Moscow wanted to “simultaneously deny it and at the same time to glory in it”, he said, adding the nerve agent was chosen “to show that it is Russia”.
In addition to the expulsions, the UK has announced measures that include increased checks on private flights, customs and freight as well as a boycott of this year’s Fifa World Cup in Russia by ministers and the Royal Family.
On Wednesday, the White House said it “stands in solidarity” with “its closest ally” the UK and supported its decision to expel the Russian diplomats.
President Donald Trump’s spokeswoman accused Russia of undermining the security of countries worldwide.