American Telephone & Telegraph Co. plans to offer a new satellite service to enable companies to design their own private telecommunications networks for video teleconferencing and high-speed computer communications, AT&T announced yesterday.

The proposed Skynet service underscores the growing competition between AT&T and International Business Machines Corp. as the telecommunications and data-processing marketplaces continue to converge.

IBM currently offers satellite communications services with Satellite Business Systems, the McLean-based telecommunications company in which IBM owns a 60 percent stake.

"We've not seen the Skynet details, but it doesn't surprise us that AT&T is becoming a stronger competitor in video teleconferencing and digital services for business customers," said Larry Weekly, an SBS spokesman.

The key to the new satellite offering is a joint-marketing agreement between AT&T and Vitalink Communications Corp. of Mountain View, Calif., a producer of satellite earth stations.

Prior to the agreement, AT&T had no base to produce the satellite antennae that beam up and receive satellite signals. Vitalink, a young venture capital company, and AT&T will jointly market and install these satellite dishes on customer sites. This will enable companies to create their own national or regional network of satellite links.

The company will seek permission from the Federal Communications Commission to launch the service and expects to begin operations by the middle of next year.

"We can now provide you with the earth station segment, the space segment and tie you into our terrestrial network," said AT&T spokesman Richard A. Brayall.

However, there are concerns about the growth potential and profitability of the business satellite communications market.