NBC, a unit of Comcast Corp. (CMCSA.O), said it has sold $1 billion in national ads for the Summer Olympics in Rio de Janeiro as the network prepares to broadcast a rare games where some of the most high-profile contests will air live in prime time for U.S. audiences.

NBC said in a statement Tuesday that the games, which start Aug. 5, were on pace to generate the biggest Olympics advertising sales ever. The $1 billion figure includes national broadcast, cable and digital ad sales, the network said.

"We've surpassed the $1 billion mark four months ahead of [the 2012 summer games in] London," Seth Winter, NBC Sports' executive vice president of advertising sales, said in a statement.

By nature of a short time zone difference between Brazil and the United States, the Rio games will feature more live events such as track and field and swimming than in London over 17 consecutive nights in August.

U.S. advertisers have already bought out some inventory, but some prime-time advertising is still available on the main NBC broadcast network and cable channels, according to NBC.

The network has not yet revealed its streaming plans for online viewing but said digital ad sales were healthy. Live streaming of events at the 2014 winter games in Sochi generated tens of millions of dollars in advertising.

NBC signed on two years ago to pay $7.65 billion for the right to air six Olympic Games from 2022 to 2032 before any other U.S. media company could bid.

NBC and rival networks have been stocking up on live sports content, which is popular with advertisers because large audiences watch in real time and cannot skip the TV commercials.

"The Olympics’ ability to dominate prime time for 17 consecutive nights is unmatched," Winter said.

Official Olympics sponsors spend significant amounts on ad time from NBC, with some even signing exclusive deals that keep other advertisers out of certain categories for the entire games.

Shares of Comcast were down 0.3 percent at $59.90 in morning trading.

Sponsors have been announcing their rosters of athletes in recent weeks. Citigroup (C.N) said this month it was sponsoring gymnast Gabby Douglas, beach volleyball's Kerri Walsh Jennings and rugby's Carlin Isles.

Visa Inc. (V.N), a global Olympics sponsor, is backing swimmer Missy Franklin and fencer Ibtihaj Muhammad, the first Muslim-American to compete in a hijab.