Two senior U.S. congressional Republicans said on Tuesday they were troubled by reports of computer hacking attacks on trading systems run by Nasdaq OMX Group Inc and asked for some answers.

The news that the NASDAQ Stock Market's computer network has been repeatedly penetrated during the past year is troubling to say the least, said House Financial Services Committee Chairman Spencer Bachus and committee member Scott Garrett, both Republicans, in a joint statement.

We would appreciate hearing from you regarding what you are doing to ensure the ongoing integrity and security of exchange trading systems and clearinghouses in response to these reports, they said in a letter addressed to regulators and the heads of major exchange groups.

(Reporting by Kevin Drawbaugh; editing by Andre Grenon)