Russia said the United Kingdom's allegation that it's responsible for a nerve agent attack is "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 UN Vasily Nebenzya said at the meeting on the poisoning incident in Britain on March 4.

Former spy Sergei Skripal, 66, and his daughter Yulia, 33, are in critical condition in hospital after being exposed to a nerve agent.

On Wednesday, Britain expelled 23 Russian diplomats, the biggest expulsion from Britain in 30 years, and canceled all high-level bilateral contact with Russia. It said it would not send government ministers or members of the royal family to attend the World Cup in Russia this year.

Britain's Deputy UN Ambassador Jonathan Allen said Skripal and his daughter were poisoned with Novichok. Allen said it is a weapon that cannot be manufactured without the use of the highest-grade state labs.

Moscow dismissed the accusations as "fairy tales".

Russian Foreign Minister Sergei Lavrov said Moscow will soon expel British diplomats in response to Britain's punitive actions, Xinhua reported.

Kremlin spokesman Dmitry Peskov also rejected the "unfounded accusations" of Russian involvement in the attack, CNN reported.

He urged other countries to use their "common sense" as they consider whether there is any proof to the claims.

Russian columnist Viktor Marakhovsky said on Thursday that internet users around the world - from average Joes to Julian Assange - are giggling at May and wondering what proof, if any, she has.

Marakhovsky said the British government had cheated the world out of the truth by giving no evidence.

Beijing on Thursday said it hopes Moscow and London can resolve their differences in accordance with the facts and the merits of the issue.

Foreign Ministry spokesman Lu Kang said China hopes they will strictly adhere to the UN Charter and principles of international laws and properly handle relevant differences through dialogue and communication on the basis of mutual respect and equal consultation.

Russia and the United Kingdom are two influential global nations and are permanent members of the Security Council of the UN, Lu said, adding that "it is very important for the permanent members of the Security Council to maintain communication and cooperation".

US Ambassador to the UN Nikki Haley said the United States stands "in absolute solidarity" with Britain.

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

Zhou Jin in Beijing contributed to this story.