Russia on Tuesday denied the United Kingdom’s allegations that it was involved in the nerve agent attack on a former spy in Salisbury earlier in March, The Guardian reported. Russia summoned the United Kingdom’s ambassador to the foreign ministry in Moscow.

Moscow’s statement came after British Prime Minister Theresa May asked it to explain why a Russian-made nerve agent was used in the attack. May said she will have to conclude there has been an “unlawful use of force” by Moscow if they do not respond by Tuesday night.

“Russia is not responsible,” the country’s foreign minister, Sergei Lavrov, said, adding that Moscow will not comply with a Tuesday midnight ultimatum that May has set. The minister said Russia’s requests to see the samples of nerve agent had been turned down which, he claimed, violated the chemical weapons convention that outlaws the production, stockpiling, and use of chemical weapons. He said under the convention, Moscow has 10 days to reply to an official accusation.

“Russia is ready to cooperate in accordance with the convention to ban chemical weapons if the United Kingdom will deign to fulfil its obligations according to the same convention,” he added.

The attack

On March 4, Sergei Skripal, a former Russian double agent, and his daughter Yulia were found unconscious on a bench outside the Maltings shopping centre in Salisbury. Both of them, and a police officer who first responded to them, are said to be in serious condition in hospital. Police said the substance that rendered them critically ill was a nerve agent.

UK health officials on Sunday asked hundreds of people who visited a restaurant and a pub where the attack took place to wash their clothes and wipe personal items like handbags and phones, in case the substance came in contact with them.

Toxic nerve agents enter the body through nose or mouth and attack the nervous system, subsequently stopping it from working.