Sony Ericsson, the world's fourth-largest mobile phone maker, reported Wednesday that it sold more than 100 million handsets last year and it's now focused on expanding its product lineup in the United States.

Sony Ericsson, a joint venture between Sony and Ericsson, shipped 30.8 million handsets in the fourth quarter ended Dec. 31, which is an 18% increase from the same period the previous year.

The company's sales for the quarter were 3.8 billion euros (about $5.6 billion) and remained unchanged from the same period last year. Sony Ericsson said its sales stayed flat because it shifted to selling lower-priced phones.

Sony Ericsson, however, didn't stick to its low-priced model when it introduced the W910 Walkman and K850 Cyber-shot models. For example, the K850 is priced at $500.

Both phones became available in selected markets last October and they support third-generation cellular technology known as HSDPA, which makes downloading music and other content a lot faster.

The K850 has a 5-megapixel camera and advanced features such as photo fix for automatically fixing the light balance to create good-quality pictures. The W910 can be used for music, 3-D games, video, and high-speed Internet access.

Sony Ericsson has about 9% of the worldwide handset market share, according to research firm Gartner. But the company is determined to knock Motorola out of the No. 3 spot.

"Our target remains to become one of the top three players in the industry, and the momentum we established in 2006 and 2007 makes this a realistic and achievable ambition," said Dick Komiyama, president of Sony Ericsson, in a statement.

The company's ambition became clear last year when it was introducing new phones practically on a monthly basis. Sony Ericsson sold more than 100 phone models last year, the company said. Then at last week's Consumer Electronics Show in Las Vegas, Sony Ericsson added some more Walkman music phones to its portfolio, as well as a clamshell phone enhanced with a diamond design for a luxurious appearance.

Walkman phones are among Sony Ericsson's most popular. Out of 145 million music-enabled phones the company has sold to date, 57 million were Walkman phones.

A big part of Sony Ericsson's strategy is to get more phones into the hands of mobile users in the United States, where several of its Cyber-shot and Walkman models are doing well, according to the company. Sony Ericsson has had a small presence in the United States, only holding 2% of the market share, compared with leader Motorola, which has 31% of the U.S. market.