Yahoo said Monday it will have a Web-based system in place to buy online ad space across some 600 newspapers and other online sites as soon as July.

The system is designed to let publishers quickly find available ad space on their own sites for advertisers, and when none is available, on other sites. The system is "almost ready" and will be launched in the third quarter of this year, Yahoo said.

The announcement comes as Yahoo is expected to respond to Microsoft's renewed threat on Saturday to pursue a proxy battle if Yahoo doesn't agree to an acquisition within the next three weeks.

The new ad system, which Yahoo calls AMP and was formerly known as Project Apex, is likely one of the technologies Microsoft is eager to incorporate into its own operations.

Microsoft's justification for its $44.6 billion bid for Yahoo on Feb. 1 was largely centered on trying to invigorate its online advertising operations, which have trailed Google's.

Yahoo's early preview of AMP may also be a move designed to show the company's strength in order to force Microsoft to raise its bid. Yahoo has maintained Microsoft's offer undervalues the company.

But Yahoo's AMP already faces a competing up-and-running service: Google's PrintAds, which lets customers who are already buying contextual Web-based ads to also place ads in around 600 daily and weekly U.S. newspapers. PrintAds also offers ad-design tools.

AMP will be the technology platform that will leverage a historic ad revenue-sharing agreement Yahoo made in November 2006 with U.S. newspaper publishers.

As part of the agreement, Yahoo provides search services, places job ads on its own HotJobs site, and sells Web advertising. The deal was expanded one year ago, and now Yahoo says 600 U.S. newspapers are part of the Newspaper Consortium.

AMP will link together the ad inventory of those publishers, offering advertisers the ability to buy search, display, local, mobile or video ads, Yahoo said.

Publishers have a couple problems with the way they sell ads now, Yahoo said in a preview of the system. When an advertiser approaches and wants, for example, 2 million impressions for a campaign advertising a new car, the publisher must use their ad systems to find out if they can deliver that many impressions. The process can take up to 15 minutes, which Yahoo describes as slow.

If the publisher can't deliver that many impressions, they must look to other Web sites to share in the deal. That typically involves phones calls, which starts another slow process by another publisher to see if they have inventory and can deliver a certain number of impressions, Yahoo said.

AMP wraps up the ability to see others' available ad space as well as a publisher's own through a Web-based interface, speeding up the ad placement process. Yahoo also describes it as a stock market for ads, with competitive bidding, as well as the ability to do behavioral, demographic and geographic targeting.