The French government disclosed yesterday that it has initiated a modest military supply relationship with Nicaragua at a time when the United States is seeking to push the revolutionary Sandinista government deep into isolation and to halt what Washington asserts is a dangerous arms buildup in Central America.

The State Department had no immediate reaction to the announcement in Paris of the $15.8 million contract signed in December, but it appeared to represent a new potential flash point of conflict between Washington and the Socialist government of President Francois Mitterrand, which has strongly supported the Reagan administration's tough approach to the Soviet Union but just as strongly dissented from American policies in the Third World.

French Defense Minister Charles Hernu said in an interview here yesterday shortly before the formal announcement of the contract with Nicaragua that France would continue to try to establish better military and economic relations with revolutionary governments, "and we would hope that America would welcome it rather than criticize it. It is better that these countries turn to us for help than to turn to others," he added in an evident reference to the Soviet Union.

Hernu explained the sale of two patrol boats, two Alouette 3 helicopters and a dozen trucks to Secretary of Defense Caspar W. Weinberger in a meeting yesterday and will probably discuss it again with Secretary of State Alexander M. Haig Jr. today. During the hour-long interview with The Washington Post yesterday, Hernu also made these points:

* He offered the clearest indication yet given that France is currently conducting tests of enhanced radiation warheads by saying that his government "will continue the studies and tests of the neutron bomb" that have been decided upon, and said that Mitterrand is studying the question of production and deployment of the controversial battlefield nuclear weapon.

* Hernu acknowledged that France quietly has begun offering proposals to its European partners to open discussions on "modifications and new definitions" for the Atlantic Alliance, which has been troubled in recent years by differing European and American reactions to the Soviet invasion in Afghanistan, peace initiatives in the Middle East and the imposition of martial law in Poland. But he said that the proposals, which were made in an effort to improve the cohesion of the alliance rather than to call it into question, were "not yet ripe for real discussion."

* Arriving in Washington four days after completing an agreement in Cairo to sell 20 Mirage 2000 fighter-bombers to Egypt, Hernu said he had come away convinced that Egyptian President Hosni Mubarak, while continuing the policy of peaceful relations with Israel, would also seek "better understanding, and diplomatic and intellectual help from moderate Arab countries" after the Israeli evacuation of the Sinai Peninsula is completed April 25.

The Reagan administration has charged that the Sandinistas are smuggling arms to the leftist guerrillas in El Salvador and are embarked on a major military buildup that threatens the entire region. Washington not only has cut off economic aid to Nicaragua but also has made a point of leaving open the impression that it is contemplating military action against the country.

Nicaraguan Foreign Minister Miguel D'Escoto said that his government has sought to buy military equipment from the United States and other European countries, but had been turned down until the French contract was signed. "The United States won't even sell to us things that are not classified as military, so we are looking elsewhere," D'Escoto said in a telephone interview.

The Ministry of External Affairs told reporters in Paris that France would provide "defensive" arms that cannot be exported to other countries under the terms of the contract. A press spokesman at the Elysee Palace said that the contract was worth about $15.8 million. Hernu disclosed here that the contract called for the training of about 10 Nicaraguan pilots and technicians in France.

Saying that one of his first official appointments in Paris after leaving Washington would be with the defense minister of Marxist-ruled Ethiopia, and noting that France and Algeria have reached new economic agreements after a period of estrangement, Hernu recalled that Soviet troops had been expelled from such "client states" as Egypt and Somalia in the past.

"From a military point of view, developing countries are very aware that they have to avoid total dependence for their security on just one country, one of the two big powers," Hernu said. "We can discuss military arrangements with countries without posing ideological conditions. We have a liberty of action that the United States should encourage," since the alternative for most of these countries would be to become totally dependent on the Soviet Union, he suggested.

In his talks with Haig and Weinberger, Hernu was expected to underscore the continuing French nuclear buildup that parallels the Reagan administration's strategic rearmament program. The key figure in persuading the Socialist Party when it was in opposition to endorse France's independent nuclear force and to steer clear of unilateral disarmament campaigns, Hernu has stressed continuity in defense policy since the Socialists swept to power last spring.

The Socialists have increased the defense budget by 17 percent and are putting nearly one-third of the expenditures into the nuclear striking force that was at the root of Charles de Gaulle's quarrel with, and withdrawal from, the North Atlantic Treaty Organization's military establishment in 1966.

Hernu said that France was attempting to modernize its nuclear force "to solve our defense problem of time. The United States and the Soviet Union can have military concepts of defending space, since they are not on each others borders. But in Europe, where frontiers are so close, that concept doesn't exist. We have to think in terms of speed of response, of measuring our defense in time. Thus we are budgeting for everything that can gain us time. One of the things I am most interested in studying here in Washington is the question of security of communications, a subject the Reagan administration is also emphasizing."

Hernu's budget contained funds for the development of a new ground-to-ground mobile ballistic missile, and for the short-range Hades missile, which could transport a neutron warhead if France decides to deploy enhanced radiation weapons.

The government has not acknowledged that it has tested a neutron warhead. Hernu said yesterday, however, "As far as study and tests go, we are ready. As far as production and deployment go, the president is still studying the file. The only decision that has been taken is to continue the studies and the tests. Technically, we know we can produce them. But that decision has yet to be made."