WASHINGTON (CNN) -- With the White House already facing sharp questions about whether top presidential advisers improperly used a Republican National Committee e-mail server to send messages that later disappeared, spokeswoman Dana Perino acknowledged Friday that millions of White House e-mails also may have disappeared.

"I wouldn't rule out that there were a potential five million e-mails lost," Perino told reporters.

The comments came in the wake of a blistering new report from a liberal watchdog group, Citizens for Responsibility and Ethics in Washington (CREW), alleging that over a two-year period official White House e-mail traffic for hundreds of days has simply vanished -- in possible violation of the federal Presidential Records Act.

"This story is really now a two-part issue," CREW's Melanie Sloan told CNN. "First there's the use of the RNC e-mail server that's inappropriate by White House officials, and secondly we've also learned that there were, between March of 2003 and October of 2005, apparently over 5 million e-mails that were not preserved and these are e-mail on the regular White House server."

Perino stressed there's no indication the e-mails were intentionally lost, but she was careful not to dispute the outside group's allegations. "I'm not taking issue with their conclusions at this point," she said. "We're checking into them. There are 1,700 people in the Executive Office of the President."

-- CNN White House Correspondent Ed Henry