Digital currency exchange operators Coinbase and Bitfinex are reporting problems with service through their websites, frustrating traders seeking to cash in on the latest surge in the value of bitcoin and other cryptocurrencies.
Wallet-provider Coinbase's website showed "service unavailable" early on Tuesday US time, flashing a message that said it was down for maintenance. Its exchange gdax.com was still quoting prices, although it also said it was experiencing a "minor service outage".
Bitfinex, another cryptocurrency exchange, tweeted it was under heavy distributed denial of service (DDoS) and its application programming interface was down.
DDoS attacks have been common on the internet, using hijacked and virus-infected computers to target websites until they can no longer cope with the scale of data requested.
It was not immediately clear if the two incidents were related to any cyberattacks.
Bitfinex last Thursday tweeted that it had been under significant denial of service attack for several days, and that the attack had recently worsened.
Bitcoin exchanges and wallets have a history of being hacked, and security experts say they become more vulnerable to cyber-crime as valuations rise.
There have been at least three dozen heists on exchanges that buy and sell digital currencies since 2011, including one that led to the 2014 collapse of Mt. Gox, once the world's largest bitcoin market.
The latest attack came last Thursday, when a Slovenian cryptocurrency mining marketplace, NiceHash, said it lost about $US64 million ($A90 million) worth of bitcoin in a hack of its payment system.
Bitfinex did not immediately respond to a request for comment. Reuters was unable to contact Coinbase since the website was down.