NEW YORK (Reuters) — Ad time for the 2004 Super Bowl is mostly spoken for and prices for the coveted football game are rising, but it may be tougher to fill the remaining commercial slots in a soft television-buying environment.

Advertisers are holding off on so-called scatter buying of other television ad time in the short-term, after pouring millions more dollars into upfront commitments for the fall program season last May, they said.

While the Super Bowl reels in top viewership year after year, the slack market could further hurt the top six U.S. broadcast networks already hit by weak ratings and threats to their ad formats from commercial-skipping technology.

“The Super Bowl is distancing itself from network prime time by the sheer fact it’s holding the largest rating of the year, whereas prime time is declining,” said Tim Spengler, executive vice president and director of national broadcast at media buyer Initiative Media, a unit of Interpublic Group of Companies.

About 80 percent of the advertising slots had been sold ahead of the Feb. 1 broadcast in Houston, Texas, said a spokesman for CBS, a unit of Viacom. Prices run as high as $2.3 million to $2.4 million for a 30-second spot, up from $2.2 million last year when the National Football League championship game was on ABC. But the remaining 20 percent, 10 to 15 commercial spots, could still prove a challenge to fill.

“There have been other years where the game has been sold out already at this point,” Spengler said. “How many people have got 2 million plus sitting in abeyance to jump in … There’s still work to be done with regard to the Super Bowl.”

With the U.S. economy poised for growth, advertisers wanted to lock in their ads early and were willing to pay more for that. Media buyers said that ad time in the first half of the game and most of the third quarter had been snapped up.

Advertisers traditionally prefer commercial spots early in the game, which is rarely close enough to keep audiences riveted through the last quarter, but after perennial stalwarts such as Anheuser-Busch (which has 10 commercial spots), PepsiCo, and General Motors made commitments, media buyers said, the number of new marketers willing to pay such prices is slim.

Consumer-products maker Procter & Gamble makes its first foray into the Super Bowl this year and is holding an internal playoff among eight brands vying for the costly spot.

Time Warner Inc. unit America Online hopes to break through the clutter by sponsoring the half-time show, along with several commercials, after airing just one 15-second spot the previous year.