Mobile online advertising spending is expected to jump 62 percent worldwide in 2012, with the United States overtaking Japan as the top market, a research firm said Wednesday. The report by eMarketer said spending on mobile advertising will reach $6.43 billion in 2012, driven by 97 percent growth in the United States, which will account for some $2.3 billion of the total. Japan had been the largest market for mobile ads until now. \"Mobile advertising is more mature in Japan, which means growth is far lower than in North American markets,\" the company said in a statement. \"eMarketer estimates spending on mobile Internet ads in Japan will grow 27.2 percent to $1.74 billion in 2012, versus 35.4 percent growth in 2011.\" Mobile is by far the fastest growing segment of Internet advertising as more consumers access the Web by smartphone or other mobile devices. \"Mobile is an exciting, fast-growing category, but it\'s still a very small piece of the overall ad pie,\" eMarketer says. \"Mobile advertising accounted for less than one percent of total ad spending worldwide last year -- and it will be a long time before it challenges other mainstay global advertising channels like TV, print and Internet ads.\" The survey found that Asia-Pacific was the largest region for mobile ad spending, with $2.56 billion, ahead of North America at $2.4 billion and Western Europe\'s $1.3 billion. A separate survey by eMarketer released Wednesday showed overall Internet ad spending up 21 percent in 2012, expected to total $107.3 billion. It found the Internet accounted for one in five ad dollars spent worldwide. The United States remains the largest market for online ads, with 2012 spending estimated to be $39.5 billion, followed by Japan at $9.6 billion and China with $7.36 billion. The research firm said China would overtake Japan in 2013 as the second largest market for all ad spending, and become the number two Internet ad market by 2014. It projected total media ad spending to grow five percent this year globally to $505 billion. A separate survey released in June by the Interactive Advertising Bureau found some $5.3 billion was spent on mobile digital advertising in 2011.