France has agreed to sell rockets and shoulder-fired rocket launchers to Nicaragua as part of an arms deal already criticized by the United States when it was revealed without public mention of the rockets, according to diplomatic sources.

The previously unknown part of the sale further highlights the sharp differences between President Francois Mitterrand's government and the Reagan administration over how best to deal with Central America. Despite Mitterrand's strong support of a tough approach to the Soviet Union, his Socialist government has expressed sympathy and support for Latin American revolutionary movements such as the Sandinistas in Nicaragua, saying they will avoid communist allies only if they find help in the West.

This was the main explanation for the $15.8 million sale signed secretly in December and revealed Jan. 7. At that time, the French Foreign Ministry insisted that the equipment was purely "defensive" and confirmed reports that it consisted of two Alouette 3 helicopters, a pair of coastal patrol boats and a dozen military trucks.

Nothing was said in public of the rockets or rocket launchers. French sources said Paris informed the United States privately that they also were part of the deal, but other diplomatic sources said the information was withheld.

A State Department official said he did not believe the French discussed the sale in detail with State, but noted that in a meeting between Defense Secretary Caspar W. Weinberger and his French counterpart, Charles Hernu, who visited Washington Jan. 7 and 8, the French-Nicaraguan deal apparently was discussed in some detail.

Defense Department spokesmen contacted Wednesday said the Pentagon had known about the sale in advance of the Weinberger-Hernu meeting and that the secretary "expressed his displeasure with the sale" to the French, but they were unsure whether the U.S. side had been told that the sale included the rockets.

The rocket launcher, a bazooka-like weapon, has become a favorite of guerrillas around the world because it can knock out armored vehicles or reinforced buildings from a distance. French firms have two models in mass production, with the STRIM 89 used by the French Army and several foreign armies.

Sale of such weapons is considered more sensitive than the previously mentioned equipment because it is more difficult to describe them as defensive. In addition, the Reagan administration charges that Nicaragua is transferring arms to rebels fighting the U.S.-backed junta in neighboring El Salvador, and individual weapons such as the rocket launchers are more easily moved.

The French government, however, says it has guarantees under terms of the sale that its weapons will not be transferred or used by others.

The French position that the sale is aimed at keeping Nicaragua from moving into a closer relationship with the Soviets and the Cubans was rejected by U.S. officials when the previously known part of the sale was announced earlier this month. It was described by one senior U.S. diplomat as "naive."

Weinberger and Secretary of State Alexander M. Haig Jr. expressed official disapproval during meetings in Washington this month with Hernu. Haig also registered a stiff complaint the following week in a meeting in Brussels with the French foreign minister, Claude Cheysson, and U.S. Ambassador Evan Galbraith told lower ranking officials in the Foreign Ministry and presidential palace of Washington's irritation.

French officials, nevertheless, said the U.S. reaction was not as strong as they had expected. Although U.S. diplomats described the session between Haig and Cheysson as tough, for example, their French counterparts portrayed the secretary's complaint as short and pro forma.

One explanation for Washington's moderate response then is the attention being devoted to the Polish crisis, with the Reagan administration eager to enlist as much French and other European support as possible for moves against the Soviet Union.

Another is the small size of the arms deal. Nevertheless, the deal is considered symbolically important here. Cheysson and Mitterrand's Socialist Party have a history of solidarity with national liberation groups such as the Sandinistas. A number of French officials have expressed the conviction that the Reagan administration is committing serious errors in Central America.

In this spirit, a French diplomatic source said, Paris also is studying potential overtures to Cuba. Cheysson and his advisers believe Cuba is part of Central America's problems, the source added, and should be brought into any attempt to solve them.