Russia, at a UN Security Council meeting, has accused the British government of inventing a "fake story" and warned that it was "playing with fire" by blaming Moscow for the poisoning of a Russian former spy and his daughter on British soil.
The Russian UN diplomat warned "we have told our British colleagues that they are playing with fire and they will be sorry".
Yulia Skripal was recovering in hospital and released a statement on Thursday, saying her "strength was growing daily". Her father remained critically ill but stable.
Nebenzia said the accusations were "horrific and unsubstantiated" and claimed the UK was waging a "propaganda war" against Russia.
He said Novichok -- the nerve agent used in the poisoning -- was "not copyrighted by Russia, in spite of the obviously Russian name" and was developed in many countries.
"It's some sort of theatre of the absurd. Couldn't you come up with a better fake story?" he asked.
She said Russia came under suspicion for several reasons, saying it had "a record of conducting state-sponsored assassinations" and that it "views defectors as suitable targets for assassination".
Pierce told delegates that Russia's request to visit Yulia Skripal had been passed on and "we await her response".
The UN meeting came amid an escalating diplomatic crisis between Moscow and the West as 60 expelled US envoys left Russia on Thursday. Over 20 countries expelled Russian envoys in solidarity with the UK, following Britain's initial expulsion of 23 Russian diplomats.
On Wednesday, Moscow failed in its efforts to persuade the Organization for the Prohibition of Chemical Weapons (OPCW) to allow a joint UK-Russia investigation into the attack.
The OPCW is already carrying out an independent investigation at the UK's request and expects to receive the results of its analysis within a week. Britain is also carrying out its own inquiry with support from the OPCW.