By MARY LOUISE WILKINSON

Two Miami exiles who were killed after landing near a heavily populated Havana suburb were on a mission to assassinate Fidel Castro, the Cuban government claimed today.

The incident occurred late Sunday near the Comodoro Yacht Club in suburban Miramar, when Commandos L, a Miami-based action group, put Sandalio Herminio Diaz and Armando Romero ashore from a 23-foot boat, the Cuban communiqué said.

Tony Cuesta, 39 - year - old group leader, and Eugenio Zaldivar Xiques were captured after being seriously wounded in a gunfight 10 miles off the coast. Two other crewmen, listed only as "Guillermo" and "Roberto" (alias Cara Vieja), were missing - and presumed drowned.

In Miami, where Cuesta has lived since 1960, his wife said she had no further information about the fate of her husband.

"I hope and pray he is all right," said Mrs. Cuesta. "But regardless of what happens, we must continue the fight against Castro. I knew before he set out that the operation was risky."

According to the Cuban Interior Ministry communiqué Commandos L launched the infiltration attempt from Marathon. The Castro officials made their usual claim that the group was sponsored by the American government.

"The objective, according to the prisoners' confession, was to assassinate the prime minister in order to create conditions favorable for an imperialist aggression," the communiqué claimed.

Government-controlled newspapers in Havana carried pictures of material allegedly seized from the boat, including hand grenades, plastic explosives, submachine guns and anti-Castro leaflets.

The infiltration try came immediately after Castro announced an island-wide combat alert against "imperialist aggressors" following a series of incidents at the U. S. Naval Base at Guantanamo in which an armed Cuban soldier was shot and killed.

Last year, Commandos L teamed up with the Cuban Referendum in Exile (RECE), sponsored by rum millionaire Jose M. Bosch, to carry out a series of attacks against Cuba.

However, Ernesto Freyre, of RECE, denied knowledge of the Sunday raid, stating, "I am sorry but there is nothing I can say."

In November, the two groups joined with the 30th of November Movement here to strafe a police station on the Havana waterfront. Three years ago, Cuesta led a Commandos L raid against the Russian freighter Baku in a Cuban port which prompted a Soviet protest note to Washington.

Sunday's infiltration attempt came on the heels of a claim by the Second Front of the Escambray-Alpha 66 that they raided a naval past at Tarara Beach, same 20 miles east of Havana, on May 19 and slipped back to a "secret Caribbean base" without losing men or equipment.

Despite the latest failure, exile activists here appeared to intensify plans for future anti-Castro action.

Manuel Antonio de Varona, former Cuban prime minister and head of the Rescate movement, flew here from his exile home in New York to coordinate plans for an action group merger that reportedly included the Second Front.

Varona, who served briefly as head of the now-defunct Cuban Revolutionary Council, said, "We must not give Castro a breather. There should be well-coordinated actions from outside to encourage the people inside to work toward overthrow of the dictator."