Russia denied the United Kingdom's allegation that it's responsible for a nerve agent attack as "completely unacceptable" at a UN Security Council emergency meeting on Wednesday.

"The Russian Federation thinks it is completely unacceptable to launch unjustified accusations as contained in the letter from Theresa May dated March 13 to the Secretary-General of the United Nations," Russian Ambassador to the United Nations Vasily Nebenzya said at the meeting on the poisoning incident in Great Britain on March 4.

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

On March 4, Sergei Skripal and his daughter Yulia were found unconscious on a public bench in the town center and were taken to a hospital by emergency services, where they remain in very serious condition, according to the UK.

Britain's Deputy UN Ambassador Jonathan Allen said the Skripal, a 66-year-old former Russian spy who became a double agent for Britain, and his 33-year-old daughter were poisoned with "novichok".

Allen said it is a weapon that cannot be manufactured without the use of the highest-grade state labs.

He said British Police Officer Nick Bailey also was exposed and remains hospitalized in serious condition, and hundreds of others in the country may have been exposed to the agent.

US Ambassador to the UN Nikki Haley said the United States stands "in absolute solidarity" with Britain. "The United States believes that Russia is responsible for the attack on two people in the United Kingdom using a military-grade nerve agent," she said.

Russia dismissed the accusations as "fairy tales".

Ma Zhaoxu, China's ambassador to the UN, said China took note of the statements made on the incident.

"We hope that a comprehensive, objective and impartial investigation could be conducted based on facts and in accordance with relevant international rules and reach an evidence-based conclusion that can stand the test of facts and history," Ma said. "We hope relevant parties could properly handle this issue through appropriate channels."

The White House said in a statement on Wednesday that the US supports Britain's decision to expel Russian diplomats.

Prime Minister Theresa May said on Wednesday that?Britain will expel 23 Russian diplomats. In her statement to Parliament, May announced the potential freezing of Russian state assets that pose a security threat and new laws to counter hostile state activity.

The Russian Foreign Ministry said Moscow would swiftly retaliate against the British measures, Reuters reported.