Websites of exchange operators Nasdaq and BATS have been attacked by hackers over the last 24 hours, causing ongoing disruptions for those trying to use the sites, spokespersons for the companies said on Tuesday.

Only the banner carrying the logo at the top of the web page for www.nasdaq.com was visible during repeated attempts to access Nasdaq's site on Tuesday afternoon. Trading in Nasdaq-listed stocks has not been affected, however.

A Nasdaq OMX Group spokesman confirmed that it had been suffering a denial-of-service attack (DOS) since late on Monday but said they had not located the origin of the attack and were unable to say when it would be resolved.

Another site, www.nasdaqtrader.com, was also affected.

The website wasn't hacked, nobody got any information. What they did was try to block access for our users, said Nasdaq spokesman Joseph Christinat.

The Nasdaq's websites are separate from their trading systems and Christinat said the breach had not affected trading in any way.

We experienced intermittent service disruptions on our corporate website and we are working to resolve the issues, he said.

Similarly, a BATS spokesperson said the company was affected by a denial of service incident. Our trading systems were not affected and there were no exchange customer disruptions associated with the incident, the exchange said in a statement.

The Nasdaq and the New York Stock Exchange have been the victims of attacks in the past. Hackers who infiltrated the Nasdaq's computer systems in 2010 installed malicious software that allowed them to spy on the directors of publicly held companies, Reuters reported last year.

A federal investigation into the 2010 cyber attack on Nasdaq found lax security practices, according to people with knowledge of the probe.

Last October, the NYSE's website was inaccessible for 30 minutes, according to an Internet monitoring company, but the exchange said there was no interruption of service.

The incident followed a video posted on YouTube, which claimed to be from the activist hacker group Anonymous.

It said the NYSE website would be erased from the Internet in sympathy with the then-ongoing Occupy Wall Street protests in Lower Manhattan.

Neither the origin of the attack nor the video could be verified by Reuters on that occasion.

A spokesperson for U.S. exchange DirectEdge said its websites have not been hit by attacks this week.

(Reporting By Edward Krudy and Caroline Valetkevitch)

(Corrects year in paragraph 10 to 2010, instead of 2011)

NASDAQ OMX Group Cybercrime