The announcement Wednesday drew a sharp response from Microsoft, which warned that a Yahoo-Google partnership would cover some 90 percent of online advertising.

The test is expected to last up to two weeks and will be limited to no more than three percent of Yahoo search queries, Yahoo said in a statement. The program will deliver Google ads alongside Yahoo's own search results.

The Sunnyvale, California-based company said the test will apply only to traffic from yahoo.com in the US and will not include Yahoo's network of affiliate or premium publisher partners.

Yahoo said the testing does not necessarily mean it will "join the AdSense for Search program or that any further commercial relationship with Google will result."

Yahoo is also fighting off a takeover bid from software giant Microsoft, arguing the offer undervalues the Internet firm.

Brad Smith, Microsoft's general counsel, said in a statement: "Any definitive agreement between Yahoo and Google would consolidate over 90 percent of the search advertising market in Google's hands."

He added: "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."

Smith said Microsoft remains committed to a 44.6 billion dollar takeover offer for Yahoo.

"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," he said.