CBS joined other US TV networks in pulling all advertising during its initial news coverage of the terrorist attack. Overall, US TV networks are believed to have lost $320m (£218.4m) in ad revenues last week.
Last week's tragic events have added to the media industry's troubles as it struggles to keep on top of the worldwide slowdown in advertising.
Viacom said it expects to grow slightly this year but that earnings before interest, taxes, depreciation and amortisation would grow by single figures rather than the double-digit growth earlier forecasts predicted.
Viacom began a $2bn (£1.4bn) share buy-back scheme in February and has said it will continue with the plan. US companies are said to be considering share repurchases or expanding existing schemes to shore up confidence in their companies and in US markets.
Mel Karmazin, Viacom chief operating officer, said that uncertainty among advertisers will hurt media companies which were already waiting for signs of a recovery.
"Most companies we spoke with believe they will return to normal advertising levels but are uncertain about the timing," Karmazin said.
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