The allies of jailed Russian opposition leader Alexei Navalny said Monday they had been unable to locate him for six days, and that he had likely been transferred.  

Navalny is serving a 19-year prison sentence on extremism charges and a court this summer ordered to move him to a harsher "special regime" prison colony. 

"We still do not know where Alexei is," his spokeswoman Kira Yarmysh said on social media. 

She said his lawyers tried to visit Navalny at the IK-6 where he was held, and tried a "special regime" colony -- IK-7 -- in the same region.

"They have just been informed simultaneously in both colonies that he is not there," Yarmysh said. 

"They refuse to say where they have taken him," she said in another social media post. 

His disappearance comes as President Vladimir Putin announced he will run for a fifth term in March, intent on prolonging his long rule. 

The opposition leader's allies believe he was taken out of the Vladimir region. 

"He can be anywhere," said another Navalny ally, Leonid Volkov.   

"What's worse, he could be transferred for several weeks," he added. 

Russia's massive prison system often takes weeks -- or even months -- to transfer prisoners between far-flung facilities by rail. 

Navalny has had his contact with the outside world seriously restricted in recent months and has been in and out of isolation cells this year.  

US 'deeply concerned' about Navalny  

The United States said it was concerned about the absence of news on Navalny's whereabouts.  

"We are deeply concerned about these reports," National Security Council spokesman John Kirby told journalists aboard Air Force One. "He should be released immediately."