After more than a year of high-level diplomatic efforts, the Reagan administration appears to have failed in its bid to win broad support from the Western European allies for its policies in Central America.

In recent weeks, as Washington has described the crisis in the region, particularly in El Salvador, in increasingly urgent terms, the administration has been faced with increasing evidence that European politicians are becoming more outspoken in their criticism of U.S. policy.

In Washington, Pentagon sources said NATO would hold its first major naval exercise in the Gulf of Mexico and Straits of Florida March 8-18. Details on Page A22.

The latest hint of allied discord came today when Belgian Prime Minister Wilfried Martens backed away from official U.S. suggestions that he had endorsed American policy in El Salvador when he met Wednesday on behalf of the 10-nation European Community with President Reagan in Washington.

After the meeting, Reagan said that "we found great agreement and support with regard to our position in El Salvador and a recognition of what is at stake there."

Although Martens, standing at Reagan's side, offered no disagreement, upon returning to Brussels today he told journalists that he and Foreign Minister Leo Tindemans had "explained exactly what our position was." Martens added: "Mr. Tindemans explained again this morning that we did not approve as such the U.S. policy."

Despite American lobbying, only one Western European government--Britain--has accepted Salvadoran President Jose Napoleon Duarte's invitation to send observers to monitor voting in El Salvador's March 28 election.

Although U.S. missions are reluctant to go into detail on the lobbying efforts, one diplomatic insider said the administration's attempts to enlist European governments in sending observers had been to little avail. "They got nowhere and most European governments let these entreaties fall off them like water off a duck's back," he said.

In recent weeks at least six countries that had been lobbied--Greece, Denmark, Italy, the Netherlands and now Belgium, as well as Canada--have publicly refused to send observers.

In Washington, the Organization of American States this week agreed to send a three-member observer delegation to the elections. A State Department spokesman said 12 countries have expressed interest in sending observers, but only five have announced publicly that they will do so--Egypt, Uruguay, Costa Rica, Colombia and Britain.

France's Socialist government was never asked because of its public disapproval of U.S. policy, including a joint declaration with Mexico in August recognizing the guerrillas of the Farabundo Marti Liberation Front as a representative of the Salvadoran people and calling for negotiations.

From one end of Western Europe to another, U.S. policy in Central America is also creating problems for governments.

In Britain, Prime Minister Margaret Thatcher has been criticized harshly by opposition politicians during parliamentary question time for her support of Reagan policies. Her government announced Wednesday that it will send two observers to El Salvador for the elections.

In Italy, where the government has said little about Reagan's policy on Central America, Socialist leader Bettino Craxi, who opposes it, has been so critical of that silence that some observers question how long he will keep the Socialists in the fragile, five-party coalition of Christian Democratic Premier Giovanni Spadolini.

At the same time, in the view of some European observers, U.S. policy has given the Soviet Union and European Communists an opportunity to divert attention from Poland.

Although Socialists in and out of power have taken a leading role in opposing Reagan on Central America, perhaps nowhere else in Europe is the criticism as loud as in France.

From President Francois Mitterrand down, France's Socialists have preached the need for a third--and thus European--"force," at least on the political and diplomatic levels, to separate the combatants in Central America and help the United States find a political solution.

Some critics have derided the activism of French policy as a way of keeping the left-wing fringe of the Socialist Party busy with a good cause in a part of the world that is only marginally important to France. Others charge that Daniele Mitterrand, the president's wife, has a personal interest in El Salvador, with her involvement in political and humanitarian organizations dealing with Central America.

Whatever the Reagan administration's impatience with the Mitterrand government, U.S. officials are perceived as having played down their displeasure. In part, that thesis reflects the administration's satisfaction with strong French backing for its tough anti-Soviet views, which have made the Mitterrand government the most reliable European ally on that score.

However, U.S. officials are known to be angry with French decisions--made without prior consultation--to recognize the Salvadoran guerrillas and sell $15.8 million in arms to Nicaragua, as well as to send aid to Vietnam.

As one senior European diplomat put it, "We admire how France manages to win the gold medal as best ally while taking the position it does on El Salvador."

At their latest meeting, Secretary of State Alexander M. Haig Jr. and French Foreign Minister Claude Cheysson appear to have misunderstood each other.

The French said Haig described French-U.S. differences over Central America as "peanuts" and thus unimportant. But the Americans claim that Haig was trying to tell Cheysson that France's interest in the region was "peanuts" compared to that of the United States.

However, even French officials who want to believe there are no serious government-to-government problems with Washington worry that the French position on Central America will reawaken lingering U.S. doubts about France's reliability as an ally at the very time when relations are the best in a generation.

One French insider said, "We could be reviving the bad memories with the American people of General de Gaulle's 1966 speech in Phnom Penh." From the Cambodian capital the then-president warned the United States of the pitfalls of its growing military involvement in Indochina and pleaded for a negotiated settlement.

Among a number of important French government and Socialist delegations that have visited Central and Latin America recently, Socialist Francis Gutmann, the secretary general of the Foreign Ministry, who had had no diplomatic experience, recently returned from the region.

This week Jacques Huntziger, the Socialist Party's chief foreign affairs official, and Nicole Bourdillat, its Central American specialist, left on visits to Nicaragua, Costa Rica, Panama and Cuba.

The announcement of the visit to Cuba followed reports in the newspaper Le Monde that Mitterrand had written to President Fidel Castro, apparently in hopes of arranging the release of longtime political prisoners, especially poet Armando Valladares.

Such a gesture could lead to an improvement in Cuba's relations with France, which the French claim Castro wants to lessen his dependence on Moscow. Any long-term breakthrough, the French insist, would depend on a settlement in Namibia, which in turn might prompt Cuba to withdraw its troops from Angola.

Although that appears far off, a Socialist official said the visit to Cuba was "not just to inaugurate chrysanthemums," the French equivalent of cutting ribbons. He said France wanted to pinpoint differences and know where the Cubans stood.

Some French officials envisage an important tradeoff for France if relations with Cuba improve: assurances that Cuba would keep hands off Martinique, Guadeloupe and other volatile French island possessions in the Caribbean as well as French Guiana on the South American coast where France maintains a major missile-firing range.

The limits of French--and European--endeavors in Latin America were illustrated earlier this week when a meeting in Caracas of the Socialist International scheduled for later this month was abruptly postponed.

The postponement apparently was because the host party, Venezuela's Democratic Action, refused to receive the Sandinistas of Nicaragua on the ground that they were not Socialists but Marxist-Leninists.

Lionel Jospin, the French Socialist Party's secretary general, had been counting on that meeting to give respectability to the Nicaraguans, whom the French have been trying to help, according to some critics.