San Francisco - Twitter says it is buying MoPub, a mobile-focused advertising company, as it works to expand its advertising reach ahead of an expected initial public offering that could come as soon as this year.

San Francisco-based Twitter Inc. did not give a purchase price. MoPub says that Twitter will invest in its core business and the company plans to continue to build its tools and technology.

Twitter says it plans to use MoPub's technology to build real-time bidding into its advertising platform. The process lets advertisers buy online ads in real time. Twitter says the technology will make it easier for its advertisers to automate their ad buying process.

Research firm eMarketer estimates that Twitter will have $950 million in worldwide advertising revenue in 2014, up from $582.8 million this year.