Youbit announced on its website that it had been hacked on Tuesday, causing a loss worth 17 percent of its total assets. The South Korean exchange didn't elaborate on the exact amount, but said all customers' cryptocurrency assets would be marked down to 75 percent of its value, and that it was stopping trade to minimize customer losses.

The cryptocurrency exchange had been hacked once before in April when nearly 4,000 Bitcoins were stolen in a cyber attack that the country's spy agency linked to North Korea, according to a South Korean newspaper report on Saturday.

An official at Korea Internet & Security Agency (KISA), the state agency that responds to cyberattacks, said the police and KISA officials were starting an investigation into the hacking.

Ban amid warnings

Youbit is a smaller player in South Korea's cryptocurrency market, with the world's busiest cryptocurrency exchange Bithumb accounting for about 70 percent of the country's market share.

The news comes as South Korea last week banned its financial institutions from dealing in virtual currencies such as Bitcoin, as the cryptocurrency soars in a bubble fueled by retail speculators, many of them from the country.

South Korea has emerged as a hotbed for cryptocurrency trading, accounting for some 20 percent of global Bitcoin transactions. About one million South Koreans, many of them small-time investors, are estimated to own Bitcoins, and demand is so high that prices for the unit are higher than in other markets.

But trading in the virtual currency has been overshadowed recently by a series of warnings from politicians and financial experts about risks.

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Axel Weber, chairman of the Swiss bank UBS, warned this week that anyone keen to invest in Bitcoin risked losing their money. "We as a bank have consciously warned against this product because we do not assess it to be valuable or sustainable," he said.

Denmark's top central banker later weighed in, saying people buying Bitcoin should not blame regulators if they suffered losses when the market crashed because it was an unregulated market. He said putting money into bitcoin was basically gambling.

Never-ending rally?

Despite the warnings, Bitcoin traded at around $18,800 (€15,925) on Tuesday morning, maintaining its global rally this year that saw prices soar from $1,000 in January to almost $20,000 now.

This comes as the virtual currency made its debut on the world's largest futures exchange on Sunday evening, when the Chicago Mercantile Exchange (CME) became the second exchange after the CBOE to offer Bitcoin derivatives trading The January 2018 contract initially spiked above $20,000 but later dropped back, having been originally priced at $19,500.

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The Youbit hack was also the second within just two weeks, Bitcoins worth $64 million were stolen by hackers who broke into the Slovenian-based bitcoin mining marketplace NiceHash.

The hack was "a highly professional attack with sophisticated social engineering" that resulted in approximately 4,700 Bitcoin being stolen, said NiceHash's head of marketing, Andrej P Škraba.

Bitcoin exchanges and wallets have a history of being targeted, and security experts say they become more vulnerable to cybercrime as valuations rise.

uhe/nz (Reuters, dpa)