The Russian Embassy in London said it would not respond until it had received samples of the chemical used
A deadline has passed for Russian officials to explain a chemical weapon attack in Salisbury in England, with the UK and Russia seemingly at a stalemate.
Prime Minister Theresa May had given the Russian President Vladimir Putin until the end of Tuesday to confirm whether former spy Sergei Skripal and his daughter Yulia were poisoned in a state attack, or if Russia had "lost control" of the nerve agent used in the attempted murder.
The Russian Embassy in London, however, said it would not respond to Mrs May's deadline until it had received samples of the chemical used in the attack.
Possible retaliation from the UK could include sanctions, although the Russian Embassy in London claimed plans were "being developed to strike Russia with cyber weapons".
It warned that "any threat to take 'punitive' measures against Russia will meet with a response".
"The British side should be aware of that," it said.
"Britain must comply with the Chemical Weapons Convention which stipulates joint investigation into the incident, for which Moscow is ready."
Allies including the US, France, Germany, and countries bordering Russia - Latvia, Estonia and Lithuania - have lined up behind the UK to express their support.
As the deadline drew closer, the British Foreign Office said: "[Boris Johnson] has emphasised that if this was a direct act by the Russian state then it would not simply be a threat to the UK, but a clear violation of the chemical weapons convention, a breach of international law and a threat to those who abide by the rules-based international order as a whole."
Mr Skripal, 66, is a former Russian spy who sold secrets to MI6 before being jailed in Russia.
He was released in a spy swap in 2010 and has lived in Britain since.
He lived an apparently quiet life until he and his 33-year-old daughter were found slumped on a bench in a shopping centre on 4 March - an attack Mrs May said was "highly likely" carried out by Russia.
Mr Skripal and his daughter remain in a critical condition in hospital.
A police officer who was among the first on the scene was also seriously affected but is recovering well.
In total, 38 people required treatment for the effects of nerve agent novichok.
Meanwhile, the UK's broadcasting regulator Ofcom has written to the Russian state-owned television channel RT as part of an investigation to assess if it is fit and proper to hold a broadcast licence.
In a statement, it said: "This letter explained that, should the UK investigating authorities determine that there was an unlawful use of force by the Russian State against the UK, we would consider this relevant to our ongoing duty to be satisfied that RT is fit and proper."