President Donald Trump, French President Emmanual Macron, German Chancellor Angela Merkel and UK Prime Minister Theresa May issued a rare joint statement Thursday, saying, "There is no plausible alternative explanation" to the attack which took place March 4 on English soil.

They added that Moscow's refusal to respond to Britain's "legitimate request" for an explanation only "further underlines its responsibility."

"This use of a military-grade nerve agent, of a type developed by Russia, constitutes the first offensive use of a nerve agent in Europe since the Second World War. It is an assault on the United Kingdom's sovereignty and any such use by a state party is a clear violation of the Chemical Weapons Convention and a breach of international law. It threatens the security of us all," they charged.

On Wednesday, Prime Minister May announced the expulsion of 23 Russian diplomats from Britain over the incident.

The attack unfolded on March 4 when former Russian double agent Skripal, 66, and his 33-year-old daughter, Yulia, were found unconscious on a bench in Salisbury, England, poisoned by a Russian-made nerve agent.

The attack sparked a public clash between Russia and the UK, with May threatening "economic war" after Moscow refused to answer her questions.

"Their response has demonstrated complete disdain for the gravity of these events," the prime minister said in an address to the House of Commons. "They have treated the use of a military-grade nerve agent in Europe with sarcasm, contempt and defiance."

"There is no alternative conclusion other than the Russian state was responsible for the attempted murder of Mr. Skripal and his daughter," May concluded.

"This represents an unlawful use of force by the Russian state against the United Kingdom," she said. "It must therefore be met with a full and robust response."

May also told Parliament there would be increased checks on private flights. In addition, she said members of the royal family and politicians would not attend the World Cup in Russia.

Undaunted, Russian officials are hitting back, saying Moscow doesn't "speak the language of ultimatums" and warned they would soon take retaliatory measures against the UK.

"We were given an ultimatum and requested in 24 hours to admit that we committed a crime. In other words, confess," the BBC quoted Vasily Nebenzya, Moscow's ambassador to the United Nations.

"We do not speak the language of ultimatums. We do not use that language with anyone. And we will not allow to be spoken to in that language either," he said.

"We demand that material proof be provided of the allegedly found Russian trace in this high-resonance event," Nebenzya added. "Without this, stating that there is incontrovertible truth is not something that we can take into account."

Meanwhile, the Guardian reports that Skripal remains in a critical condition.

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