UNITED NATIONS -- In the latest sign of rising tensions between Britain and Russia over the poisoning of a former Russian spy, the two countries' envoys clashed at a UN Security Council emergency meeting convened especially over the matter on Wednesday.

The exchange of barbs came shortly after British Prime Minister Theresa May announced that her government will expel 23 Russian diplomats, cancel high-level bilateral contacts with Russia, and freeze Russia state assets "wherever we have the evidence that they may be used to threaten the life or property of UK nationals or residents."

"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," Russia's UN ambassador Vasily Nebenzya said.

"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 added.

The London-Moscow row started following the murder attempt on former Russian double agent Sergei Skripal and his daughter Yulia in Britain's southwestern city of Salisbury.

Found unconscious on a bench outside a shopping center there on March 4, they remain in critical condition.

Britain claims the father-daughter pair was exposed to a nerve agent and Russia is responsible for the act, which Moscow denies.

At the UN Security Council meeting, Britain's Deputy UN Ambassador Jonathan Allen said hundreds of his countrymen had potentially been exposed to the agent.

He said 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, a chemical nerve agent that cannot be manufactured without the use of the highest-grade state laboratories.

"This was no common crime," he said. "It was an unlawful use of force."