Russia overnight NZT dismissed accusations of any involvement in the poisoning of an ex-spy and his daughter as "nonsense," saying it will only cooperate with a British investigation if it receives samples of the nerve agent believed to have been used.
Police, meanwhile, said the investigation of who poisoned Sergei Skripal and his daughter, Yulia, will last many weeks and that they are not ready to identify any persons of interest in the inquiry.
The father and daughter remain in critical condition in a Salisbury hospital.
British Prime Minister Theresa May has said Russia's involvement is "highly likely," and she gave the country a deadline of midnight Tuesday (1PM Wednesday NZT) to explain its actions in the case.
She is reviewing a range of economic and diplomatic measures in retaliation for the assault with what she identified as the military-grade nerve agent Novichok.
US and European officials were quick to offer words of support for Britain, which will need the backing of its allies if any new sanctions are to have any impact.
"It sounds to me that they believe it was Russia and I would certainly take that finding as fact," said US President Donald Trump.
He said Washington will condemn Moscow if it agrees with Britain's findings, adding that he would discuss the incident with May.
James Nixey, head of the Russia program at the Chatham House think-tank, said Britain must offer more than a symbolic response.
"Will actions meet with responses which have real-world effects?" he said. "Or are we going to have more fudge?"
Russian Foreign Minister Sergey Lavrov told reporters in Moscow that his country's requests to see samples of the nerve agent have been turned down.
He insisted that Russia is "not to blame" for the poisoning.
"We have already made a statement to say this is nonsense," he said. "We have nothing to do with this."
It was not immediately clear if Lavrov's comments constituted Russia's official response to May's ultimatum.
Meanwhile the cases of other Russians who have died under mysterious circumstances are being raised. British Home Secretary Amber Rudd said police and the domestic security service will look into 14 deaths in Britain that might be linked to Russia.
"In the weeks to come, I will want to satisfy myself that the allegations are nothing more than that," Rudd said.
"The police and MI5 agree and will assist in that endeavor."