An ally of jailed Kremlin critic Alexei Navalny told supporters on Wednesday to move to using cryptocurrencies to help fund their cause as a crackdown threatens to make donations illegal. Russia has moved to ban Navalny"s Anti-Corruption Foundation and regional groups as "extremist", dealing a blow to the network built up by President Vladimir Putin"s most outspoken opponent. It is illegal to donate to "extremist" groups. A court is next due to consider whether to approve a prosecutorial request to ban Navalny"s movement on June 9. For years, he and his allies have crowdfunded donations from their supporters via their foundation"s website and through other platforms, including using bitcoin, which cannot be traced by Russia"s authorities. On Wednesday, Leonid Volkov, a close Navalny ally who is based abroad, said it was vital for the group to continue raising funds, but that they needed to protect their supporters. The group"s regular donation point on its website was unavailable on Wednesday and appeared to have been taken down. Volkov did not say anything about such a move. "It"s not so easy now - the movement of money within the Russian banking system is clear and transparent to the Russian state, and therefore you and I need to get away from it," Volkov said in comments on YouTube. "Since the state is squeezing us out of simple and understandable collection of donations through the normal banking system, it means we will patiently teach everyone to use cryptocurrencies, and we will learn ourselves," he said. Navalny is serving a 2-1/2-year jail sentence for parole violations he calls trumped up.