Fox News Channel chairman and CEO Roger Ailes responded to Keith Olbermann's latest critical volley against Bill O'Reilly on Monday, saying the MSNBC host's behavior "is over the line."

Ailes, appearing Monday at the summer meeting of the Television Critics Association, was referring to a weekend incident at the gathering in which Olbermann whipped out a mask of O'Reilly and gave a Nazi salute.

Ailes said Olbermann picks on Fox's O'Reilly to boost his ratings.

"Clearly he has no viewers except those he gets when he attacks Fox News," Ailes said.

Of Saturday's incident, Ailes said, "I really think that's over the line."

During his "Countdown" show on MSNBC, Olbermann regularly tweaks O'Reilly. Olbermann has named O'Reilly his "Worst Person in the World" at least 15 times. The nightly "award" is Olbermann's way of criticizing what he deems bad behavior.

O'Reilly has referred to Olbermann — although not by name — as a "notorious smear merchant" and pointed out the low ratings for "Countdown."

"The O'Reilly Factor" on Fox is No. 1 in the cable TV news ratings. Olbermann generally runs third in the same time slot.

Also Monday, Ailes said Fox will mark its 10th anniversary with a "Thank You America" tour this fall. Shows fronted by O'Reilly, Shepard Smith, Greta Van Susteren, Sean Hannity and Alan Colmes, and Neil Cavuto will do live versions of their shows from 10 cities around the country, including Boston, Chicago, Dallas and San Diego.