(Reuters) – Investors should consider risks associated with virtual currencies, including bitcoin, before trading in them, two U.S. regulators warned on Tuesday. The warnings are the latest in a string of advisories from U.S. state regulators, including Nevada and Maryland, as they look to toughen rules on the use of the controversial cryptocurrency. The value of virtual currencies is highly volatile and the concept behind the currency is difficult to understand even for sophisticated financial experts, Andrea Seidt, president of the North American Securities Administrators Association (NASAA) president, said in a statement on Tuesday.