Secretary General Javier Perez de Cuellar says he has installed a "radar" on the United Nation's 38th floor that is scanning the world for signs that "differences are in danger of becoming a real conflict."

This figurative early warning system has quickly homed in on at least two disputes in which, the secretary general said in an interview, "I have already started direct but quiet diplomacy."

One involves the border skirmishes between Nicaragua and Honduras. On Oct. 8, Perez de Cuellar said, he called the foreign ministers of the two countries up to his office and "I surprised them. I put them in a room alone and I told them, 'Here you are. Keep discussing. Don't fight too much.' And then I left.

"An hour and one-half later, everybody was smiling. At least it was a beginning. It created a friendly atmosphere."

Last last month, he followed up that effort by formally putting his "good offices" at the disposal of the two countries.

Perez de Cuellar also has intervened in the territorial dispute between Venezuela and Guyana. "They now know someone is watching," said a U.N. official involved in the talks. "Of course, they are still stating their public positions, but by getting to them early in the game, he has foreclosed the option of force."

Perez de Cuellar, 62, cited these two examples to show he was not just talking theory when he dropped a diplomatic bombshell in September, warning in his first annual report that the United Nations was failing in its prime task of keeping the peace. In that report, he suggested new ways in which both he and the Security Council could be more effective in using the institution as a negotiating forum to avert crises and to resolve those that do erupt.

Since that time, the Peruvian diplomat has won praise for his courage and support for his proposals from the vast majority of foreign ministers who participated in the General Assembly's annual policy debate.

"They were supportive," he said, "but they didn't add anything."

This tossed the ball back at Perez de Cuellar, and he has set up a Secretariat task force "to see what we can extract for their comments," and to spell out options for making the United Nations more active in dispute settlement.

In his report, Perez de Cuellar drew a line between actions he could take himself and those that required cooperation from a governments -- both the 15 council members and those involved in disputes.

"I don't need their benediction for my prerogatives under the U.N. Charter," he said. He explained that in addition to his own efforts at negotiation, he plans to dispatch fact-finding teams of U.N. officials to the scene of disputes, "people who will report to me, so I can report to the council -- accurate reports on the basis of which the council can act."

It would only be at this point, he said, where "I will need help" in the form of cooperation from council members.

To make the process work, the secretary general is convinced that disputants must be brought -- kicking and screaming if necessary -- into the council, once the fact-finding process is complete.

Asked how governments could be forced to take their disputes to the United Nations, Perez de Cuellar noted that all U.N. members have signed the charter, "but have conveniently forgotten that it binds them to settle their disputes through negotiation." He suggested that acceptance of this principle by the big powers would make it difficult for virtually any nation to refuse such a summons.

At present, the council deals with disputes that are before it largely through private consultations which often degenerate into rhetorical exchanges of formal statements, and by public meetings that are more of the same. Perez de Cuellar suggested that to transform itself into a true negotiating forum, the council must accept radical changes in this format, and bring the disputing parties into the closed negotiations that now include only council members.

Should a U.N. peace force be required to defuse differences, he said, "the idea of guaranteering support for its mandate needs to be refined."

"If the five permanent members give a mandate to a U.N. force, and the force is brushed aside [as Israeli troops did in Lebanon in June and Turkey did on Cyprus in 1974], they should have some reaction. We should expect the permanent members to feel responsiblity, take some measures to give respectability to the U.N. force, to make a strong warning which will deter countries from military action. That is what we understand by guarantees."

The five permanent members of the Council are the United States, the Soviet Union, China, Britain and France. The British and French have backed his ideas, Perez de Cuellar said.

American Ambassador Jeane J. Kirkpatrick has applauded the secretary general's criticisms of the United Nations and supported his proposals in principle.

But in a discussion of the issue taped for the Voice of America, she backed away quickly when he specified the need for big-power guarantees, and even U.N. sanctions, if peace-keeping mandates are violated. His implication was that Washington should have taken stronger action when Israel circumvented the U.N. peace force in Lebanon.

Consultations on the secretary general's concepts are now underway among some council members. But Perez de Cuellar is known to believe that an assembly resolution might do more harm than good if it is too specific and too politicized. He said he would rather pursue the matter informally.

"I am a very patient man," said the secretary general, "but if I hear nothing from the council in the next two or three months, I am entitled to take new initiatives."

The United Nations, he added, is already the "appropriate forum" to negotiate disputes, including the long conflict in the Middle East. "Dinner is served, but nobody sits at the table."