Jonathan Brady PA/AP A police officer at a cordon near the Maltings in Salisbury, England, Friday, March 30, 2018, near where Russian former spy Sergei Skripal and his daughter Yulia were found on a bench after they were attacked with a nerve agent. (Jonathan Brady/AP)

The daughter of a former Russian double agent who was poisoned alongside her father is awake and alert, stoking hopes that she could offer details crucial to cracking a case that has inflamed diplomatic tensions between Moscow and the West.

The condition of Yulia Skripal, 33, who was poisoned by a military-grade nerve agent, is rapidly improving, doctors said. Her father, Sergei Skripal, 66, who was also poisoned, remains in critical but stable condition.

In a statement Thursday evening, the National Health Service branch in England said: “We are pleased to be able to inform you that Yulia Skripal is improving rapidly and is no longer in a critical condition. Her condition is now stable.”

Christine Blanshard, medical director for the Salisbury District Hospital, said: “I’m pleased to be able to report an improvement in the condition of Yulia Skripal. She has responded well to treatment but continues to receive expert clinical care 24 hours a day.”

The BBC, citing multiple sources, said she was conscious and conversing, raising hopes that, as a key witness, she may be able to offer valuable intelligence that could help identify her would-be assassins.

Skripal’s improved condition follows the announcement this week that the United States, one of more than two dozen countries to expel Russian diplomats in retaliation for the poisoning, would kick out 60 Russians and close the Russian Consulate in Seattle. Russia responded by expelling 60 U.S. diplomats and closing the U.S. Consulate in St. Petersburg. On Friday, Russia’s Foreign Ministry summoned ambassadors from dozens of countries to inform them of further tit-for-tat expulsions.

It was unclear what, if anything, Yulia Skripal would remember in the lead-up to the events on March 4, when she and her father were found slumped on a park bench in the quiet English city of Salisbury.

The two were exposed to Novichok, a rare nerve agent developed by the Soviet Union, British authorities said. Investigators said the main concentration of the nerve agent was found on the front door of the Skripal home, which remains cordoned off.

In 2006, Skripal was jailed by Russia for selling information to MI6, Britain’s foreign intelligence service. He was released four years later as part of a high-profile spy swap.

Britain says it is “highly likely” that Russia is responsible for the poisoning. Russia denies any wrongdoing.

Earlier this week, Sergei Skripal’s niece, Viktoria Skripal, told the BBC that she had little hope her relatives would survive.

“Out of 99 percent, I have maybe 1 percent hope,” she said, adding that if they did survive, they probably would be “invalids for the rest of their lives.” She also said that the family has not told Sergei’s mother about the attack. “Our first priority was to protect our granny,” she said.

Viktoria Skripal said she did not know who was responsible for the poisoning. Speaking from Russia, she said: “Our side say it was the British secret services; the British say it was the Russians. I don’t know.”