(CNN)Tensions between London and Moscow over the attempted murder of a former Russian spy ratcheted up another level Thursday after Russian Foreign Minister Sergey Lavrov warned that a move to expel British diplomats from Russia would "absolutely" be going ahead.
Russia to retaliate 'soon' against UK with its own diplomat expulsions
Speaking at a question-and-answer forum in Moscow, Lavrov did not provide a time frame but said the expulsions would take place "soon," state media reported.
The move follows Britain's decision Wednesday to kick 23 Russian diplomats out of the country after concluding that Russia was responsible for a nerve agent attack on former Russian double agent Sergei Skripal and his daughter in England earlier this month.
It will be the single biggest expulsion of Russian diplomats from Britain in more than 30 years, Prime Minister Theresa May announced Wednesday, as she set out a raft of measures intended to target Russian assets and strengthen UK defenses.
On Thursday, Lavrov accused May of grandstanding in her response to the incident, and said the Kremlin response would "come very soon."
"You understand that as polite people, we will first inform our British colleagues of the response. Unlike them, grabbing the microphone in order to accuse Russia of everything," Lavrov said.
A spokesman for Vladimir Putin described the UK's accusations as "unfounded" and said "it won't be long until" the Russian President makes a decision on retaliatory measures.
"The offers will be considered by the Foreign Ministry, other institutions, but the main decision will be made by the President," Kremlin spokesman Dmitry Peskov told reporters Thursday. "There is no doubt he will choose the option that corresponds the most with the Russian national interests."
On Thursday, British Defense Secretary Gavin Williamson said Moscow had made a "deliberate political decision" to poison Skripal. He accused Russia of "ripping up the international rulebook" and "attempting to "subvert, undermine and influence" countries around the world.
"Russia should shut up and go away," Williamson said. "It's often described as a cool war that we are entering -- I would say it is feeling exceptionally chilly at the moment."
The two governments have been locked in a war of words since Skripal, 66, and his daughter Yulia, 33, were found slumped on a park bench in the English city of Salisbury on March 4. The pair are critically ill in the hospital after being exposed to a nerve agent known as Novichok that was developed in Russia, UK officials believe.
May had given Moscow until midnight Tuesday to explain whether the attack was directed by Russian authorities or whether the government had lost control of the nerve agent.
But the Kremlin, which has steadfastly denied the accusations, ignored the deadline and said Britain had not given it enough time to respond to May's request.
On Thursday, the UK, US, Germany and France issued a joint statement insisting Russia was responsible for the attack and that "there is no plausible alternative explanation."
The four urged Russia to "live up to its responsibilities as a member of the UN Security Council to uphold international peace and security."
"We call on Russia to address all questions related to the attack in Salisbury. Russia should in particular provide full and complete disclosure of the Novichok program to the Organisation for the Prohibition of Chemical Weapons (OPCW)," the statement read.
US President Donald Trump also weighed in Thursday, telling reporters: "It certainly looks like the Russians were behind it. Something that should never, ever happen, and we are taking it very seriously, as I think are many others."
"I spoke with the Prime Minister and we are in deep discussions," he added. "A very sad situation."
Trump's comments followed those made by US Ambassador to the United Nations Nikki Haley on Wednesday, who blasted Russia and said the Trump administration stood in "absolute solidarity with Great Britain," in the strongest US statement yet on the incident.
"If we don't take immediate concrete measures to address this now, Salisbury will not be the last place we see chemical weapons used," she said.
On Thursday, May paid a visit to Salisbury to "speak to people who responded to this terrible incident that took place," she said, and thank those who were "continuing to work hard to investigate, to get to the bottom of those who are responsible, but also to ensure that the public are reassured."
Skripal was sentenced to 13 years in prison in 2006 for spying for Britain, according to Russian state media accounts of the closed hearing.
Russian court officials at the time said he'd received at least $100,000 for his work for MI6, the British intelligence service. He was granted refuge in the UK after a high-profile spy exchange between the United States and Russia in 2010.
His daughter Yulia is thought to be one of the few members of his immediate family still alive after his wife and son died in recent years. She was visiting him from Russia at the time of the incident.
The poisoning of the pair has put further strain on UK-Russia relations that have been fractious ever since the assassination of another former Russian spy, Alexander Litvinenko, in 2006.
A UK inquiry found that two Russian agents poisoned Litvinenko at a London hotel bar in 2006 by spiking his tea with highly radioactive polonium-210, and that Putin "probably approved" Litvinenko's killing. The Kremlin has always denied the accusation.
Both countries have acknowledged the deterioration in sentiment, with Lavrov admitting in December it was "not a secret" that the Russia-UK relationship was at a "very low point."
Earlier Thursday, British Foreign Secretary Boris Johnson accused Russia of having a "smug, sarcastic response" to the UK's calls for an explanation.
Johnson also confirmed that the UK would submit a sample of the nerve agent to the Organisation for the Prohibition of Chemical Weapons for it to carry out its own tests.