NEW YORK: Although the National Football League (NFL) season is only just getting started, initial Nielsen data has shown NFL ratings fell 14% among the crucial 18- to 49-year-old age group in September.

According to Bloomberg Intelligence analysis of the latest data, ratings for CBS's Thursday night games fell 15%, while NBC's Sunday night games dropped 12%, and ESPN's "Monday Night Football" also attracted 16% fewer viewers.

The news follows worse-than-expected viewing figures for the Olympics on NBC and the US Open tennis tournament on ESPN, and may feed into wider concerns in the media industry about whether live sports can maintain its dominance.

This is especially so in an era of "cord-cutting" and as traditional TV networks face fierce competition from digital-only rivals.

Broadcasters and advertisers are unclear at this stage about the reasons for the decline in NFL viewership, although one possibility is the presidential election.

The debate between Hillary Clinton and Donald Trump on September 26 drew 84m viewers and this cut into ESPN's Monday Night Football, the Wall Street Journal reported.

"We're scratching our heads. We cannot pinpoint any specific reason why the numbers are down," said Andy Donchin, a media buyer at Amplifi US, an ad-buying unit of Dentsu Aegis Network. "It is probably being caused by a confluence of events," he added.

The drop in ratings creates a headache for TV networks because they are required to compensate advertisers if they don't deliver on the audience numbers originally promised.

These "make-goods" involve the TV networks having to offer advertisers free commercial time elsewhere and, according to Bloomberg, TV networks so far this season have missed their estimates.

It said that last year one unidentified media buyer bought 100 NFL ad spots and got six free commercials to compensate for ratings that fell short of estimates. But, this year, that same buyer expects to receive about 10 make-goods.

However, Bloomberg continued, one positive for the TV networks is that they could respond by reducing the amount of ads during other programming, so raising prices across the rest of the market.

Data sourced from Bloomberg Intelligence, Wall Street Journal; additional content by Warc staff