LONDON, UK: Advertising technology company OpenX has announced a partnership with leading Asian ad agency Dentsu as it steps up efforts to challenge Google's dominance of the online advertising market.

California-based start-up OpenX is the world's leading independent ad exchange, and the Dentsu partnership follows one announced earlier this year with France Telecom's mobile unit Orange in Europe.

OpenX, backed by Accel Partners and Index Ventures, serves more than 350 billion display ads per month. Digital market research firm comScore says 409 billion display ads were served in May in the United States.

OpenX will now seek to exploit the relationships that Dentsu's digital ad agency cci has with hundreds of Japan's top websites to expand its business in Japan, the world's second-biggest online ad market after the United States.

OpenX's technology allows publishers and advertisers to buy and sell online display ads through real-time auctions, connecting many small or niche Web publishers as well as larger players with global buyers they might not otherwise have found.

"One of the reasons why this looks like it's beginning to work is it's giving major media companies a way to get into the space," OpenX Chief Executive Tim Cadogan told Reuters in a telephone interview.

"It's the first time that we've seen a Japanese player actually get into the exchange space in this way," added Cadogan, a former advertising executive at Yahoo.

Yahoo is one of the market leaders in online display advertising, the placing of ads that occupy space on Web pages. Most online advertising takes the form of sponsored search results, as served up by Google and others.

OpenX's venture with Orange should start operations in the next couple of months, Cadogan said. "We're looking to extend this regional network of partners at least to the eight top players over time," he added.

OpenX Chairman Jon Miller, head of digital operations at Rupert Murdoch's News Corp and former CEO of AOL, said he believed OpenX could emerge as a real rival to Google in online display advertising over time.

"No one's really organised not just an alternative to Google but an alternative way of doing it," he told Reuters, citing the scalability and openness of OpenX's technology as factors that would help it succeed.

"A combination of those services with market-making local partners should be the right model."