Russia has issued a chilling warning after Britain gave it a deadline to answer accusations of involvement in a poisoning attack in Salisbury, but US and EU allies have expressed support for Britain condemning the attack.
Prime Minister Theresa May gave Russia until midnight on Tuesday to explain how a Soviet-era nerve agent was used against a former Russian double agent.
Speaking in an interview on state television, foreign ministry spokeswoman Maria Zakharova warned: "One should not threaten a nuclear power."
Ms May, who said on Monday it was "highly likely" Russia was behind the poisoning of Sergei Skripal and his daughter, won support from some of Britain's main European allies and the European Union which denounced the attack as "shocking" and offered help to track down those responsible.
Ms Zakharova also warned against any possible suspension of Russian broadcaster Russia Today (RT).
"Not a single British media outlet will work in our country if they shut down Russia Today," she said in the broadcast.
With Britain 'all the way'
US President Donald Trump said he would condemn Russia if British evidence incriminated Moscow.
In a telephone call with Ms May on Tuesday, he said he was with Britain "all the way", according to a statement from Ms May's office.
France's Emmanuel Macron and Germany's new coalition also expressed solidarity as the UK headed into a showdown with Mr Putin.
Jens Stoltenberg, the secretary-general of the US-led NATO alliance, said the attack was "horrendous".
Russia, however, signalled little likelihood it would respond adequately to London's call for a credible explanation by the deadline.
Denying it had played any part in the attack, which left the 66-year-old Mr Skripal and his 33-year-old daughter fighting for their lives, Russia said it would ignore the ultimatum until London handed over samples of the nerve agent used and complied with international obligations for joint investigations of such incidents.
"Any threats to take 'sanctions' against Russia will not be left without a response," the Russian foreign ministry said in a statement. "The British side should understand that."
Russia is in the run-up to a presidential election on Sunday in which President Vladimir Putin, himself a former KGB spy, is expected to coast to a fourth term in the Kremlin.
Mr Skripal, a former officer with Russian military intelligence, betrayed dozens of Russian agents to British intelligence before being arrested in Moscow and jailed in 2006.
He was released under a spy swap deal in 2010 and took refuge in Britain where he had been living quietly in the cathedral city of Salisbury until he and his daughter were found unconscious on a public bench there on March 4.
A British policeman who went to the aid of Mr Skripal was also affected by the nerve agent. He is now conscious in a serious but stable condition.
May promises 'robust' action
Ms May said on Monday Britain had identified the substance as belonging to the lethal Novichok group of nerve agents developed by the Soviet military in the 1970s and 1980s.
She and her ministers said Britain would take further "robust" punitive action against Russian interests — beyond sanctions already in place — if Mr Putin did not come up with a credible explanation of events.
Speaking to reporters at the White House, Mr Trump acknowledged the British charges of involvement against Russia, but said he needed to talk to Ms May before rendering a judgment.
"As soon as we get the facts straight, if we agree with them, we will condemn Russia or whoever it may be," said Mr Trump, who earlier fired Secretary of State Rex Tillerson after a series of policy rifts, said.
"It sounds to me like they believe it was Russia, and I would certainly take that finding as fact."
While trade figures show Russia accounts for less than 1 per cent of British imports, London is of major importance for Russian companies seeking to raise capital, and since the 1991 collapse of the Soviet Union has become the Western capital of choice for many Russian business leaders.
Britain could call on allies for a coordinated Western response, freeze the assets of Russian business leaders and officials, expel diplomats or launch targeted cyber attacks.
It may also cut back participation in the football World Cup which Russia is hosting in June and July.
The expression of solidarity from the EU came despite tensions over British preparations to quit the bloc next year.
The EU imposed travel restrictions and asset freezes against 150 people and 38 companies in response to Russia's annexation of Crimea from Ukraine.
EU nationals and companies are also banned from buying or selling new bonds or equity in some state-owned Russian banks and major Russian energy companies.
But diplomats in Brussels said, despite sharing Britain's anger, the bloc is unlikely to have much stomach for imposing additional sanctions on Russia since attributing the nerve attack to Moscow was difficult and keeping existing economic sanctions going was proving a strain.
Ms May said on Monday Russia had shown a pattern of aggression including the annexation of Crimea and the murder of former KGB agent Alexander Litvinenko, who died in 2006 after drinking green tea laced with radioactive polonium-210.
Moscow denied any responsibility for that murder despite the findings of a public inquiry which said it had probably been approved by Mr Putin.