Bitcoin sank further below $6,000 on Friday, heading for a second straight month of declines at around 20 percent, as demand for the largest cryptocurrency continues to suffer from security and regulatory concerns.

The digital currency slid to about $5,861 early Friday, according to composite Bloomberg pricing, touching the lowest since November. Rival coins including Ethereum, Ripple and Litecoin also retreated. All are set for declines of about 20 percent or more in June.

Confidence in cryptocurrencies is being tested this year with hacks from Japan to South Korea raising security questions. At the same time, central banks and other regulators across the globe have warned about the potential for investor losses and fraud, casting a shadow over the asset class half a year after Bitcoin won a place on Chicago derivative exchanges.

Bitcoin, which is down almost 60 percent this year, was labeled a “speculative bubble” earlier this week by Nobel economics prize winner Robert Shiller.