By Gerry Shih

SAN FRANCISCO, Nov 14 (Reuters) - Twitter Inc said on Thursday it would introduce self-serve ads for small- and medium-sized businesses in three countries outside the United States, marking one of its first moves to expand revenue as a publicly listed company.

Businesses in the United Kingdom, Ireland and Canada will be able to buy "promoted" ads that can be shown to targeted Twitter users beginning this week, the company said.

Twitter held a successful initial public offering last week that raised $1.8 billion. Its stock price has since soared, implying a market capitalization of more than $24 billion.

Twitter, which made $317 million in revenue in 2012, generates the majority of its sales through selling ad packages directly to large companies and international brands such as Verizon Communications Inc or Samsung Electronics Co Ltd . But analysts believe it has the potential to greatly boost sales by letting smaller businesses buy automated ads without the help of Twitter salespeople.

Google Inc, for instance, became an online advertising powerhouse by automating its ad-buying capabilities for small- and medium-sized business.

Twitter gave U.S. businesses early access to the self-serve program earlier this year. The company has said it intends to eventually roll out the program around the world.

Analysts expect Twitter to make more than $1.1 billion in 2014 revenue, according to Thomson Reuters data.