Moscow warned on Thursday it was preparing to retaliate against the "irresponsible" expulsion of its diplomats from London, as Britain urged its allies to take a stand against Russia over the poisoning of a former spy.
The escalating international scandal is unfolding as former Russian double agent Sergei Skripal and his daughter Yulia remain in critical condition after exposure to the Soviet-designed chemical Novichok on March 4 in the British city of Salisbury.
Britain on Wednesday said it would expel 23 Russian diplomats and suspended high-level contacts over the poisoning, which it has blamed on Russia.
British Prime Minister Theresa May said Russia could be directly responsible or may have "lost control" of the nerve agent. She had given Moscow until midnight Tuesday to disclose details of the Novichok programme to the international Organisation for the Prohibition of Chemical Weapons (OPCW).
French President Emmanuel Macron Thursday also said he would announce unspecified "measures" in the coming days, condemning the attack in the "strongest possible terms." His office earlier backed Britain's claim that Russia is behind the attack.
Kremlin spokesman Dmitry Peskov Thursday slammed Britain's position as "absolutely irresponsible".
He warned that retaliatory steps would soon follow and President Vladimir Putin would choose the option that "most suits Moscow's interests."
Russia would respond by kicking out British diplomats, Foreign Minister Sergei Lavrov was quoted as saying at an event in Moscow, adding that it would happen "soon."
Russia has rejected demands by Britain to explain how Novichok was used on British soil, accusing May of resorting to populism and using Russia as a scapegoat.
May's statements that Moscow is behind the poisoning are "completely crazy accusations against Russia, our entire country, our people," Russian foreign ministry spokeswoman Maria Zakharova told journalists Thursday.
Moscow has mostly shrugged off the measures announced by May, ridiculing the prospect of the termination of high-profile visits.
Zakharova further accused Britain of refusing to "grant access to Russian citizen (Yulia) Skripal" or to work with Moscow through the OPCW and give Russia access to the poisonous substance.
British Foreign Secretary Boris Johnson told the BBC however that Britain is "entirely in conformity" with OPCW procedures and is sending a sample of the nerve agent to the watchdog for examination.
'No other explanation'
Britain on Wednesday called an emergency meeting of the UN Security Council over the incident, winning the support of the United States as US Ambassador Nikki Haley said Russia was "responsible for the attack".
NATO allies also expressed support for Britain following the first use of a nerve agent in Europe since World War II.
In a joint statement by its 29 member states, the alliance said the attack was a "clear breach of international norms and agreements" and called on Russia to fully disclose details of the Novichok programme.
President Donald Trump has demanded Moscow "provide unambiguous answers" to explain what is believed to be the first nerve agent attack in Europe since World War II.
Johnson further appealed to international partners to "stand with us against Russia" in a Washington Post editorial, saying the poisoning is "part of a pattern of reckless behaviour by" Putin showing "reckless defiance" of international rules.
He accused Moscow of using assassinations to send a signal to dissidents that "we will find you, we will catch you, we will kill you."
Putin, who is standing in presidential elections Sunday, has not yet commented on Britain's measures. His spokesman said the poisoning row had no effect on the campaign.
Besides expelling the diplomats, suspending high-level contacts and cracking down on Russian criminals and dirty money, Britain is expected to announce additional military spending on chemical weapons defence.
Defence Secretary Gavin Williamson is due to give a speech Thursday announcing a £48 million (54 million euro) investment in a new cutting-edge chemical weapons defence centre to counter international threats, British media reported.
Russia argues that it has destroyed all of its chemical weapons and that its military chemical programme has been shut down since the mid-1980s.
Russian chemist Vil Mirzayanov, who worked in the chemical weapons programme until 1992 and had exposed the Novichok agents prior to leaving for the United States, however claimed Moscow stored the substance and its authorities "are still keeping it in secrecy."
Mirzayanov said Russia was likely behind the attack, though adding that it was also possible that somebody used the Novichok formula published in his book to synthesise the chemical.