Ms. Skripal, who lives in Russia, was visiting her father when they were poisoned, and it is not clear whether she will return.

Russian officials say they have been improperly denied access to her, but the British police said last week that Ms. Skripal was aware of the Russian offer to visit and help, and that she had turned it down.

On Tuesday, the Russian Embassy in London wrote on Twitter: “We congratulate Yulia Skripal on her recovery. Yet we need urgent proof that what is being done to her is done on her own free will.”

The Sunday Times of London, citing anonymous sources, has reported that senior intelligence officials in Britain were talking to their counterparts at the C.I.A. about sending the Skripals to the United States. The two Russians would be provided with new identities, part of a broader effort to keep them safe in case there should be another attempt on their life.

The Russian Embassy seemed to acknowledge that possibility with another post on Twitter later on Tuesday, again demanding proof that Ms. Skripal was acting of her own accord. “Secret resettlement of Mr and Ms Skripal, barred from any contact with their family will be seen as an abduction or at least as their forced isolation,” the post read.

The case has mixed high-stakes international diplomacy with elements of farce. Hundreds of diplomats around the world have been expelled, Britain has moved to crack down on the financial dealings of Russians in the country, and President Vladimir V. Putin’s government has offered a variety of alternative explanations for what happened and complained that the Skripals’ pets — two guinea pigs and a cat — were killed by the British and then cremated to destroy evidence.