France has promised the United States to delay delivery of two Alouette-3 helicopers to Nicaragua--the most controversial military equipment in the $15.8 million arms deal signed with the revolutionary Sandinista government last December--according to French government sources.

President Francois Mitterrand agreed to the delay in his brief meeting with President Reagan at the White House earlier this month, a session that dwelled on differences over Central America, the sources said.

Senior American officials have said they thought any new French arms deal with Nicaragua most unlikely. The original deal involved the two helicopters, two coastal patrol boats, 45 trucks, 100 helicopter-mounted rocket launchers and 7,000 rocket rounds.

Foreign Affairs Minister Claude Cheysson, apparently embarrassed by leaks from the Reagan administration about delays in delivery of the helicopters, issued a statement recently that skirted an official denial of the delay.

He said, "We have signed a contract with Nicaragua. We will put the contract into effect within a reasonable amount of time, which these kinds of supplies require."

Behind Mitterrand's decision to delay delivery, sources said, was the desire to avoid potentially serious damage to U.S.-French relations, which in some areas, especially defense and African policy, are the closest in the past 25 years.

Mitterrand has not abandoned his stated misgivings about American policy in Central America, but has agreed, for tactical reasons, to soft-pedal French policy initiatives for the next three or four months, according to government sources.

Mitterrand, who will host the June summit of major industrial nations, wants a free hand in organizing that meeting and realizes, these sources added, the difficulties involved in antagonizing the United States on Central America in advance of the Versailles gathering.

France angered the United States by signing the arms deal with Nicaragua, whose military buildup the United States has condemned as a threat to the region.

French officials explained the decision by saying that Nicaragua should be encouraged to diversify its sources of arms supply, which are heavily dependent on the Soviet Bloc, particularly Cuba.

French Socialists from Mitterrand down have expressed their conviction that the West would be better served in the region if European countries provided an alternative to the positions of the United States and the Soviet Union.

Many Socialists, including the president's wife, Danielle, have become personally involved with Central American affairs.

Though the Socialist government is deeply committed to an anti-Soviet policy in Afghanistan and Poland and is in accord with Washington on key questions of European defense, Central America has proven an issue that has aroused the ideals of the party.

From its beginnings last spring, the Socialist government favored providing an alternate Western presence in Central America, and Mitterrand has spoken out against U.S. policy in the Third World.