SEATTLE — Microsoft Corp. and Yahoo Inc. have agreed to make their two instant-messaging programs work together, a partnership that could threaten market leader America Online, people familiar with the situation said.

The deal was expected to be announced early today, these people told The Associated Press. One of them works closely with Microsoft; the other was briefed on the deal. Both spoke on condition of anonymity because they were not authorized to disclose details.

A Yahoo-Microsoft partnership, allowing users of the competing services to exchange messages seamlessly, would represent close to half of the instant-messaging market. Market leader AOL holds just over half the market.

Instant-messaging services are becoming increasingly vital, not just as social lifelines for young people but also in the business world.

Their providers are adding video chatting and Internet telephone functions, and Google Inc. entered the arena in September with a service of its own that includes voice chat.

"Up until now, AOL has been able to pick and choose its partners, command the royalties it wants," said Robert Mahowald, an analyst with research firm IDC. "They've moved to develop this market at their own pace. This forces them to take a more aggressive stance."

Microsoft, Yahoo and AOL all declined to comment.

AOL's instant-messaging product, AIM, had some 51.5 million unique U.S. users in September, as opposed to about 27.3 million for the competing MSN Messenger and 21.9 million for Yahoo's Messenger, according to Nielsen/NetRatings.

Other instant messaging products, such as Trillian, allow people to send and receive messages from multiple messengers.

None of the top instant messaging systems have communicated directly with each other yet, even though promises of "interoperability" have been made for years.

Redmond-based Microsoft has long sought to forge some sort of deal to elevate the profile of MSN Messenger.

The company also has been in talks with AOL over possible partnerships with Microsoft's MSN online unit, although it's not clear where those talks stand now.

"I think what's really pushing this is Microsoft's ability to see that it's long been playing cat and mouse with AIM and ICQ (another AOL instant messaging service)," Mahowald said. "Microsoft needs to stake its presence in a big way."

Microsoft already has a product that lets business users send and receive messages from Sunnyvale, Calif.-based Yahoo, AOL and Microsoft's instant messaging systems. But the product does not extend to consumer users.