The International Telecommunications Satellite Organization, the multinational consortium that provides worldwide satellite communications, said yesterday that it plans to sell its excess satellite capacity for domestic-communications use by its member nations.

The Intelsat proposal to sell surplus satellite space has drawn complaints that Intelsat will be competing unfairly with U.S. companies that want to sell satellite space to foreign countries.

In addition, Intelsat confirmed previous reports that the Soviet Union and other Eastern Bloc countries are seriously considering becoming full-fledged members of the consortium in the near future.

"I would not be surprised if serious consideration is given in the near future to an integration of memberships [between Intelsat and Intersputnik, the satellite communications organization for Soviet Bloc countries and their allies]," said Jose Alegrett of Venezuela, the Intelsat deputy director general for business planning and external relations.

However, the focal point of Intelsat's announcement is that the agency could become a major factor in domestic satellite communications in many nations and may underprice its competition.

"It is predatory pricing, and it is done to bar new entrants like us," Fred Landman, president of Pan American Satellite Co., said recently. Pan American Satellite is a New York company recently authorized by the Federal Communications Commission to provide service to Latin America.

But a spokesman from the Communications Satellite Corp., the U.S. representative to the nonprofit Intelsat, said that "any [Intelsat] prices would be cost-based [passing on the actual costs of planning and operating the satellite]. As long as that's followed, I don't think that there will be any unfair competition."

GTE Spacenet and American Satellite Co., both providers of domestic satellite systems, had no comment yesterday on the potential impact of more space being sold on the already saturated domestic satellite market.

Intelsat officials blame lower-than-expected growth in the international communications business for the excess capacity and say that selling or leasing space on the satellites could cut the cost of communications services for its members.

The sales could bring hundreds of millions of dollars to Intelsat and could take potential customers away from private companies that make and launch satellites.

Intelsat is a 110-country consortium, with the United States holding the largest single share, that carries roughly two-thirds of the world's overseas telephone traffic and virtually all international television transmissions. The Communications Satellite Corp., the U.S. representative to Intelsat, coordinates such services between Intelsat and clients in this country.

Government officials said that while they support the notion of increased utilization of Intelsat's excess capacity, there are several unanswered questions about the proposal.

Charles Loveridge, State Department spokesman for the Bureau of International Communications and Information Policy, said that while the United States "strongly supports in principle efforts to utilize Intelsat's excess capacity," the decision raises "important legal, economic and technical questions that we believe remain unanswered."

"We would support the notion of utilization of excess capacity efficiently," said Albert Halprin, chief of the FCC's common carrier bureau. "Our concern is that any system of utilizing that capacity take into account the relevant costs, and that there be enough information available and a chance to study the information to assure that it does."

Alegrett's statement about the possible Intelsat membership of the Soviet Union and other Eastern Bloc countries comes shortly after a "memorandum of understanding" was signed by the Soviet Union and Intelsat to formalize the Soviet Union's relationship to the 109-member nation global satellite consortium.

The understanding, which came after seven years of negotiation, lays the groundwork for increased use of Intelsat's network for global transmission of voice, data and television transmissions.