Russia won't take on faith any conclusion relating to the Skripal affair until such a time as Russian experts are allowed access to the samples mentioned.
Russian Foreign Minister Sergei Lavrov says ex spy poisoning could be 'in interests' of UK government
Lavrov also suggested that the poisoning of Skripal could also be in the interests of the British special forces who are known for their abilities to act with a licence to kill".
In Washington, the State Department said there was no justification for the Russian move and that the United States "reserves the right to respond."
Russia's mass expulsion of US diplomats Thursday is the latest tit-for-tat move triggered by the poisoning of a Russian former double agent in England.
Investigators had earlier sealed off the bench where the Skripals were found, a pub and restaurant the pair visited, and the grave of the former spy's wife.
Given international conventions that prohibit the use of chemical weapons, research conducted at Porton Down is defensive in nature, according to the defence ministry.
UN Secretary-General Antonio Guterres has condemned the alleged use of nerve agent in the poisoning of a former Russian spy in Britain.
Britain's foreign ministry and the Russian embassy in London have been doing battle on social media over the death of ex-double agent Sergei Skripal.
US President Donald Trump had pressed Russia to provide answers but did not suggest that Moscow had a hand in the attempted murder.
US and other allies joined London in demanding answers on how a Soviet-designed nerve agent was used in the attack against a former double agent.
The United States agreed with its ally Britain on Tuesday that Russia was probably behind an attempt to poison a former double agent, saying the perpetrators should face punishment.
Britain was to hold a second meeting of its national emergencies committee on Saturday in response to a nerve agent attack on a Russian former spy.
On Wednesday, British police confirmed for the first time that a nerve agent was used and that their probe was now an attempted murder investigation.
Pyongyang and Kuala Lumpur had unusually strong links for years, but have been set at loggerheads by the killing of the half-brother of leader Kim Jong-Un by two women using VX nerve agent.