Russian opposition leader Alexey Navalny missing after being removed from prison

Jailed Russian opposition leader Alexey Navalny's team says they don't know where he is after the prison camp where he was being held told them he's no longer there, Navalny's spokeswoman said Monday.

The spokeswoman, Kyra Yarmysh, said his lawyers tried to see Navalny on Monday at the prison but were told he's no longer listed there. They had since tried to find him at two other nearby prison camps and were told he wasn't there, either, she said.

"They are refusing to say where they have transferred him," Yarmysh wrote on X.

Navalny was being held in Correctional Facility No. 6 in the Vladimir region, about 100 miles east of Moscow.

National Security Council spokesperson John Kirby said Monday the White House is working with the U.S. Embassy in Moscow to learn more about Navalny's disappearance and is "concerned" to learn he's been gone "for allegedly a week."

Navalny's team last week said he had suffered a "serious health incident," saying he had collapsed in the solitary cell he was being held in. His team said they believed he may have fainted from hunger.

"He fell ill in his cell last week. He got dizzy and laid down on the floor. The colony staff came over immediately, lowered the cot, laid Alexey down and gave him an IV," Yarmysh wrote at the time.

"We don't know what it was, but given the fact that he's not being fed, is being kept in a punishment cell with no ventilation and the time for walks has been reduced to a minimum, it looks like a hunger faint," Yarmysh wrote.

Navalny, a lawyer-turned-politician, has been in jail since 2021 upon returning to Russia after recovering in Germany from nerve agent poisoning that he blamed on the Kremlin. In 2022, a Russian judge added another nine years to Navalny's sentence of 2 1/2 years for embezzlement and other charges.

Earlier this year, Navalny's team sounded the alarm over his deteriorating health while in solitary confinement, saying he has not received any treatment. They said he has been repeatedly put in solitary confinement for two-week stints for months.

ABC News' Justin Ryan Gomez contributed to this report.