NEW YORK (Reuters) - U.S. banks Morgan Stanley (>> Morgan Stanley) and Goldman Sachs Group (>> Goldman Sachs Group Inc) are being investigated by New York's attorney general as part of a probe into alternative trading systems known as "dark pools," according to Fox Business Network.

Morgan Stanley spokesman Mark Lake and the attorney general's office declined to comment to Reuters on the report. A spokeswoman for Goldman Sachs was not immediately available for comment.

Dark pools are broker-run trading venues that let investors trade shares anonymously and only make trading data available afterwards, reducing the chance of information leaking about trade orders. But the lack of transparency around dark pools has drawn the scrutiny of regulators, concerned that brokers and proprietary trading firms that use aggressive high-frequency trading strategies have an unfair advantage over other clients.

Barclays (>> Barclays PLC) is facing allegations by New York Attorney General Eric Schneiderman that it lied to clients about its dark pool. The bank has urged the dismissal of the lawsuit in New York.

Germany's Deutsche Bank (>> Deutsche Bank AG) and Switzerland's UBS (>> UBS AG) and Credit Suisse (>> Credit Suisse Group AG) have said they are being probed by U.S. regulators as well.

(Reporting by John McCrank; Editing by Lisa Shumaker)