Alexei Navalny supporters say they can't find him after his move in prison system

Dec. 11 (UPI) -- Attorneys for imprisoned Russian opposition leader Alexei Navalny said they have lost contact with him and don't know if he is still in a penal colony roughly 150 miles east of Moscow.

Navalny, one of the fiercest critics of Russian President Vladimir Putin and the current government in Moscow, has not been heard from after telling his supporters he was no longer at the Correctional Facility No. 6, where they had been seeing him.

His spokeswoman Kyra Yarmysh said they had since tried to find Navalny at two other nearby prison camps and were told he wasn't there.

"They are refusing to say where they have transferred him," Yarmysh said on social media.

His team said that Navalny suffered a "serious health incident" last week after collapsing in solitary confinement, possibly fainting from hunger.

National Security Council spokesman John Kirby said the White House is "very concerned" about Navalny about his disappearance.

"He should be released immediately," Kirby said. "He should never have been jailed in the first place, and we're going to work with our embassy in Moscow to see how much more we can find out."

Navalny's disappearance comes just days after Putin announced on Friday that he will seek a fifth term in office as Russia holds elections early next year.

Navalny's website urged residents to vote against Putin in the election.

"We will conduct an election campaign against the war," Navalny's website said. "And against Putin. Exactly. A long, stubborn, exhausting, but fundamentally important campaign, where we will turn people against the war."