After years of painstaking efforts, communist and Third World nations are on the verge of securing for the U.N. agency UNESCO significantly expanded powers to shape the future of global communications and international news.

At the biennial UNESCO general conference being held here, the United States and other nations with a tradition of a free press are pinned down in complicated negotiations over the full extent and precise definition of UNESCO's expanded powers. Several Western diplomats describe their objective as "damage limitation."

It now appears virtually certain that the U.N. Educational, Scientific and Cultural Organization will emerge as arbiter of what this conference has defined as a principal political issue of the 1980s: information power.

The conference has unanimously reelected as director-general Amadou Mahtar M'Bow of Senegal, a controversial but respected figure and an outspoken advocate of what UNESCO's communist and Third World majority calls "the new world information and communication order."

The meeting has also voted a 34 percent increase in UNESCO's operating budget, with greater spending earmarked for communications and information programs during the next three years.

The move to spell out for UNESCO as role as a kind of arbiter over communications media in developing countries was given impetus by the MacBride Report, drafted by a UNESCO-created international commission headed by former Irish foreign minister Sean MacBride.

The 275-page study has stirred wide controversy here with its politically explosive recommendations.

The conference appears certain to create a new international body to funnel voluntary financial and technical assistance to those developing countries wishing to set up programs to train journalists and strengthen communications and news facilities.

The United States and other Western nations have already agreed in principle to creation of the new machinery "within the framework of UNESCO." pWhat remains to be negotiated is its mandate and how much control M'Bow and UNESCO will have over it.

The West is having difficulty pressing its case because the UNESCO initiatives on the mass media have brought the Soviet Bloc and a significant majority of developing countries into a formidable alliance.

As in past UNESCO media debates, many of their speeches here accused the West of "colonialist" domination of global communications and international news.

The MacBride Report contains recommendations that appeal to both the communist preference for government-controlled media and to the developing countries' push for a greater role in news distribution and communications.

Communist nations and a solid bloc of nonaligned and developing countries also see the proposed new aid machinery as an opportunity to give UNESCO the practical arm it needs to begin trying to implement their "new world information order."

The U.S. delegation and its allies are trying to quash or at least limit the damage from a barrage of proposals that the communist and nonaligned bloc are trying to have adopted. In the view of the Western governments and of international press groups monitoring the negotiations, several of these proposals could seriously affect international news gathering and distribution. p

Some of these proposals seek to:

Establish standards for "responsible" reporting and promote the idea that the news media should serve the greater needs of social, political and economic development.

Establish an international code of ethics for journalists that would serve as a check against biased reporting, warmongering, and propagandizing in favor of racism and apartheid.

Promote some form of "international protection" for journalists, especially foreign correspondents. The MacBride Report came out strongly against this idea, warning that it could lead to a form of licensing.

Coupled with this trust into the content and gathering of news are other proposals that could directly affect the world's major wire services, which are frequently attacked at UNESCO for allegedly exercising "monopolistic" control over international news distribution.

Several countries have already expressed their support of a recommendation of the MacBride Report calling for "effective legal measures" to limit the process of concentration and monopolization" in news distribution and to define the responsiblity of international news agencies in complying with "specific criteria and conditions defined by national legislation and development policies."

Other proposals seek to deepen UNESCO's involvement in satellite communications, telecommunications tariffs, and cross-border flow of computer data.

At first glance, many of these proposals appear to be difficult to translate into practice and certainly unenforceable by a U.N. agency with no legislative powers. Past UNESCO resolutions on the mass media have posed little demonstrable threat to everyday journalistic practices.

But many of the nonaligned nations are tying to turn the resolution on the MacBride Report into a blueprint for their new information order, and Western governments are opposed to allowing proposals such as these to become part of UNESCO's operating philosophy.

M'Bow has told the United States and others that he now wants UNESCO to end divisive talk and turn to practical ways to help developing countries reduce the "Imbalances" in news and information flows between North and South.

He is now backing a compromise resolution on the MacBride Report that would eliminate almost all the more radical proposals. Several hard-line nations, however, have resisted compromise.

These negotiations are to continue Monday. Later next week will come the talks over the composition of the new aid machinery and over the make up and role of its proposed 35-nation advisory board.

The U.S. delegation and others are understood to have warned M'Bow that they will not contribute funds or technical expertise if UNESCO appears committed to propagating government control of the media.