Yahoo has announced a deal to buy video advertising platform BrightRoll for $640m to boost its ability to make money from marketing on desktop computers and mobile devices. 

"Video, along with mobile, social, and native, is driving a surge in digital advertising," Yahoo chief executive Marissa Mayer said.

"Acquiring BrightRoll will dramatically strengthen Yahoo's video advertising platform, making it the largest in the US," it added. 

Yahoo planned to serve its ad inventory on the BrightRoll platform, which it described as a "growing and profitable" business with revenue of over $100m a year. 

Yahoo said it is paying $640m in cash for San Francisco-based BrightRoll, which lets advertisers aim video ads at audiences on desktop computers, smartphones, tablets, or Internet-linked televisions. The acquisition was expected to close early next year.

BrightRoll will continue to offer its products and services after the takeover is complete, according to Yahoo. 

Yahoo's share of the billions of dollars spent this year on digital ads in the US digital advertising market is expected to drop 4.9%, having slipped 5.8% in 2013, according to industry tracker eMarketer.

Yahoo has acquired dozens of firms since Mayer became chief in the middle of 2012 and took on the challenge of transforming it from a faded search engine to a venue for premier, personalised digital content tuned to increasingly mobile lifestyles. 

"I've talked on our earnings call about the transformation to return our iconic company to greatness," Mayer said about the BrightRoll buy. 

"This acquisition is an important step in that transformation and has been accomplished in a strategically and financially compelling way," she added.