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Microsoft, Yahoo and AOL have announced an alliance in online advertising to rival Facebook and Google.

The deal will begin in January and means they can sell each others' unsold internet display ads.

The three firms say they intend to share some resources, but will keep their sales teams separate and competitive.

The companies hope that by utilising each others' resources, they can save money and increase advertising revenue.

Rik van der Kooi, from Microsoft Advertising Business Group, said in a conference call that he did not expect any problems arising from anti-trust legislation.

"The competition is only going to increase. [We] don't expect any issues on that side," he said.

Yahoo executive vice-president Ross Levinsohn said the tie-up was a "fundamental rethinking" of the internet ad market.

That could be a tall order, as Google and Facebook increasingly dominate the online ad market and are forecast to increase their share, according to research company eMarketer.

The latest figures show Facebook has a 16% share and Google 9% of the US online ad market.

In contrast, the alliance group of companies are all expected to lose ground in the US market.

The latest figures from eMarketer show a 13% share for Yahoo, 5% at Microsoft and 4% at AOL.