PASADENA, Calif.  The verbal battle in the late-night wars got louder Thursday as a top NBC executive struck back against on-air attacks on the network’s once and future “Tonight Show” host Jay Leno.

The executive, Dick Ebersol, chairman of NBC Universal Sports, said the reason for Mr. Leno’s return to NBC’s late-night roster after a short stint in prime time this season was a simple one: disappointing ratings for Conan O’Brien’s “Tonight Show.”

Referring to the pointed jokes made this week by Mr. O’Brien and David Letterman of CBS, Mr. Ebersol said it was “chicken-hearted and gutless to blame a guy you couldn’t beat in the ratings.”

He added that “what this is really all about is an astounding failure by Conan.” Mr. Ebersol is a veteran at the network, with a longstanding relationship with NBC Universal’s embattled chief executive, Jeff Zucker. Mr. Ebersol also has a deep link to the network’s late-night history, having been a creator of “Saturday Night Live,” and he has been frequently consulted on changes in NBC’s late-night lineup.