Worldwide spending on mobile advertising is expected to hit $18 billion this year, up from $13.1 billion in 2013, according to a new report by research firm Gartner.

Although video ads will experience the highest growth as a result of wider adoption of tablets, display ads will generate the most revenue from advertisers, the report says.

Despite the nearly 40 percent increase in spending this year, growth is slowing, said Gartner research director Stephanie Baghdassarian.

"Over the next few years, growth in mobile advertising spending will slow due to ad space inventory supply growing faster than demand, as the number of mobile websites and applications increases faster than brands request ad space on mobile device screens," Baghdassarian said in a press release.

Between 2015 to 2017, however, the market's growth should accelerate due to "improved market conditions," such as new ad-targeting technologies, Baghdassarian said. Gartner, which is headquartered in Stamford, Connecticut, predicts global mobile ad spending will increase to $41.9 billion by 2017.

According to the report, mobile ad spending will increase across the world, led by North America. Emerging markets in Africa, Eastern Europe, Latin America, and the Middle East will grow as they continue to adopt mobile technology. By 2015, those markets' growth rates will exceed the worldwide average, Gartner says.

As companies compete for customers' attention, the mobile ad market will continue to be robust while ads in traditional media will shrink.

"The mobile channel will become more and more integrated into 360-degree advertising campaigns, eating up budget historically allocated to print and radio advertising," Baghdassarian said.