Online company AOL Inc. is buying the high-traffic website Huffington Post in a $315-million US deal.

The acquisition, which will put Huffington Post co-founder Arianna Huffington in charge of all AOL content, brings AOL an additional 25 million unique visitors a month.

The deal "will create a next-generation American media company with global reach that combines content, community and social experiences for consumers," AOL CEO Tim Armstrong said in a statement announcing the deal early Monday.

Founded in 2005, Huffington Post is owned by Huffington, Kenneth Lerer and a group of other investors. AOL will pay $300 million US of the purchase price in cash.

Huffington will be named president and editor-in-chief of The Huffington Post Media Group, which will include all Huffington Post and AOL content, including Engadget, TechCrunch, MapQuest, Patch and more.

Since laying off a large number of staff last year, AOL has continued reorganizing its operations. The company, which separated from Time Warner in 2009, has acquired new businesses, launched and relaunched websites, and rolled out a new web advertising system.

AOL CEO Tim Armstrong said last week he sees 2011 as a "comeback year" for the internet company as it continues its turnaround efforts and starts expanding its ad business in the second half of the year.

The company's fourth-quarter results show it is still struggling to boost ad sales. AOL had just a 5.3 per cent share of the U.S. display advertising revenue in 2010, down from 6.8 per cent in 2009, according to eMarketer. At the same time, Facebook accounted for 13.6 per cent of display revenue last year, up from 7.3 per cent in 2009.

Huffington Post grew quickly from startup to online colossus and ranks as one of the top 10 current events and global news sites.

Over time, it launched city-specific pages and developed a roster of sections such as food and books. The work of its 70-person paid staff is augmented by content from news outlets and 6,000 bloggers who write for free.

The deal is expected to close late in the first quarter or early in the second quarter, pending regulatory approvals.