Yahoo is to start displaying Google search ads in a small number of its search engine queries, a move interpreted as the latest in a series of Yahoo manoeuvres to resist Microsoft's acquisition attempt.

The trial, expected to last up to two weeks and be limited to up to 3 percent of Yahoo search queries in the US, is specifically for Google's AdSense for Search service. In other words, Yahoo would be acting as one of the web publishers that carry pay-per-click text ads from Google. The ads will appear only in

Yahoo noted that "the testing does not necessarily mean that Yahoo will join the AdSense for Search programme or that any further commercial relationship with Google will result". Yahoo will not comment on the nature or timing of any potential relationship with Google.

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Microsoft, whose acquisition offer was rejected by Yahoo's board in February, on Saturday said it will launch a proxy fight to attempt a hostile takeover if Yahoo doesn't agree to the acquisition in the next three weeks.

On Wednesday, Microsoft blasted the Google-Yahoo announcement, saying that a broad outsourcing deal would inevitably run into regulatory trouble because it would give Google more than 90 percent of the search-advertising market.

"This would make the market far less competitive, in sharp contrast to our own proposal to acquire Yahoo. We will assess closely all of our options," said Brad Smith, Microsoft’s general counsel.

"Our proposal remains the only alternative put forward that offers Yahoo shareholders full and fair value for their shares, gives every shareholder a vote on the future of the company, and enhances choice for content creators, advertisers, and consumers," Smith said.

Google has a share of between 70 percent and 75 percent of US search ad spending, and Yahoo has about 15 percent, said Karsten Weide, an IDC analyst. If Yahoo fully outsourced its search ads, Google would have a monopoly in this segment of the market, but such a deal wouldn't give Google a monopoly on overall ad spending, he said.

With Yahoo's search business, Google's share of the US online ad spending would have been around 36 percent in 2007's fourth quarter, Weide said. This could be an argument against antitrust concerns, along with the fact that Yahoo would likely get most of the money per click, and that while Google rules search advertising, it is a minor player in other online ad segments, like display ads such as banners, he said. Still, it's clear that a search ad outsourcing deal would attract a lot of regulatory attention.