(Undated) -- Some owners of a volatile virtual currency are having trouble accessing their bitcoin.

The Bitcoin Foundation says two of the exchanges that act as banks for the currency have experienced a cyber-attack. According to the foundation, the "denial of service" attack has frozen transactions and temporarily halted withdrawals.

While bitcoin owners can't move their virtual money, the Bitcoin Foundation says the attack has not resulted in any theft and all funds held by the exchanges are still safe.