In the midst of the increasing tension in U.S.-Cuban relations, Cuba secretly has offered to exchange an America agent imprisoned in Havana for a Puerto Rican Woman convicted of a 1954 terrorist attack on the U.S. Congress.

Carter administration sources said yesterday the Cuban proposal, which was relayed to Washington by an East German lawyer, would involve the swap of Lawrence K Lunt, an American held by the Cubans since 1965, for Lolita LeBron, who is serving a 25-to-75-year U.S. prison term.

Because of the indirect way in which the offer was made, the sources stressed, U.S officials are uncertain about whether it is a serious proposal that has the full backing of President Fidel Castro's government.

The sources added that the matter has been referred to the White House for a still pending decision on whether the State Department should make direct contact with Havana and explore the possibilities of negotiating an exchange of the two prisoners.

Complicating the situation, the sources said, has been the deterioration in relations caused by President Carter's accusations that Cuba helped to train and equip the rebels who invaded Zaire last month.

Cuba has denied the charges emphatically; and, in an interview Monday with The Washington Post, Castro said U.S. Cuban relations are now at their lowest point since the beginning of the Carter administration.

In addition, Rep. Stephen J. Solarz (D-N.Y.), who also talked with Castro on Monday, told reporters after his return to Washington yesterday that Castro had said he couldn't release political prisoners like Lunt "under existing circumstances". Solarz quoted Castro as saying, "Nobody in Cuba would understand it".

For different reasons, Lunt and LeBron have become political symbols who excite strong emotions among various groups with the power to cause problems for U.S. foreign policy.

Lunt, 54, who has been a cattle rancher in Cuba, has admitted that he acted as a contract agent collecting information for the Central Intelligence Agency. He is beginning the 14th year of a 30-year sentence imposed by the Cubans, and his plight frequently has been cited by U.S. conservatives who oppose any moves toward better relations with Cuba.

LeBron, 57, was one of four Puerto Rican nationalists who began firing pistols at random from a spectators' gallery in the House of Representatives on March 1, 1954. The shooting spree, in which four congressmen were wounded, was intended to dramatize demands for Puerto Rican independence from the United States.

In the years since, she has become a symbol of national pride for many of the 3.2 million people on the U.S. commonwealth island. In March, when she was given a brief furlough from prison to return to the island for her daughter's funeral, thousands turned out to greet her as a national heroine.

She has never recanted her views about the need to free Puerto Rico through armed struggle; and administration sources note, to release her from prison now could create the risk of her upsurge of nationalist terrorism on the island and the mainland.

Castro repeatedly has accused the United States of practicing colonialism in Puerto Rico. As a result, the sources said, the administration has to consider whether the prisoner exchange offer was intended as a propaganda ploy to stir unrest and trouble for the United States in the Caribean.

The offer came to Washington's attention as the result of another multi-national prisoner swap that took place in April and May. That complicated transaction saw the release of an Isralei pilot, held in Mozambique; an American student imprisoned in East Germany and a convicted Soviet spy serving a term in a U.S. federal prison.

The central figure in arranging that swap was Wolfgang Vogel, an East German lawyer who has negotiated a large number of prisoner exchanges between East and West Germany and who helped engineer the return of American U2 spy plane pilot Francis Gary Powers from the Soviet Union.

Among these who dealt with Vogel in the three-way swap was Rep. Benjamin A. Gilman (R.N.Y). According to the sources, Vogel used him to pass along the State Department the message that Cuba was willing to discuss a Lunt-LeBaron swap