Amazon is buying video game streaming platform Twitch Interactive for $970 million in cash as it seeks to expand its presence in gaming.

As video gaming grows into an online spectator sport, gameplay video feeds are becoming increasingly lucrative. Twitch had 55 million unique visitors in July, most of whom went to the site to watch other people play games.

Digital video advertising in the U.S. will reach $5.96 billion this year, according to eMarketer, up 41.9 percent from 2013.

Amazon already has an in-house gaming studio that makes games, and its Fire TV set top box was designed to attract gamers.

Google had been in talks to buy Twitch, but no deal materialized.

Amazon's purchase is set to close in the second half of 2014.