Microsoft Corp. and America Online Inc. both say they support a world where people can fire off "instant messages" to their friends on the Internet no matter what software the sender or recipient is using. But when it comes to making that happen, the two technology titans remain hopelessly divided.

Although the firms have offered approaches to resolving their tense standoff over instant messaging, the two sides moved no closer yesterday to settling the conflict.

AOL succeeded early yesterday in blocking the fifth version of Microsoft's messaging software. A Microsoft official said the company was working on developing another version of its software to get around AOL's latest defenses.

The controversy over instant messages--which appear on a recipient's computer screen almost instantly after they are sent and allow real-time typed communication--began last week when Microsoft introduced its MSN Messenger software, which allowed people to reach not only other users of Microsoft's technology but also the 40 million people who use AOL's messaging software. Previously, both sender and recipient had to use the same type of software to be able to trade instant messages.

Accusing Microsoft of making an "unauthorized intrusion" into its data network, AOL electronically jammed incoming messages from Microsoft users. Microsoft responded by releasing new versions of its software, which AOL quickly moved to thwart.

AOL wants Microsoft to stop releasing software that attempts to circumvent AOL's network barriers and instead sit down and talk about the issue. Microsoft, however, believes its software is the quickest way to promote "open standards" on the Internet. It detailed its argument in a letter it sent to an AOL executive on Monday, responding to an earlier AOL missive.

"Imagine a world in which a user of one telephone service could not talk to a user of another telephone service or one in which a user of one e-mail service could not send e-mail to a user of another e-mail service," Microsoft Vice President Brad Chase wrote. Chase urged AOL to participate in a panel set up by the Internet Engineering Task Force, a consortium of technology specialists and businesses, to develop a common standard for instant-messaging technologies. Although AOL is the market leader in instant messaging, it has not actively participated in the process.

AOL said yesterday that rushing forward with a standard could introduce some of the problems consumers face with e-mail, such as unsolicited correspondence, into the instant-messaging world. "There are real practical, technical, operational, security and privacy issues that need to be dealt with as you put together open standards," said Ken Lerer, a consultant to AOL.

Lerer said AOL was committed to providing consumers with "interoperability," but he would not say whether AOL supported a common "open" technological protocols that any firm could use to write messaging software.

Industry executives voiced concern that AOL would move slowly through the open-standards process out of fear that eventual agreement on a standard could lead to many of its users defecting to rival software products.