WASHINGTON (AP) _ A Cuban-born man who pleaded guilty to conspiracy in the 1976 assassination of former Chilean Ambassador Orlando Letelier was sentenced Thursday to 12 years in prison.

U.S. District Judge Aubrey Robinson Jr. called the deaths of Letelier and an aide, Ronni Moffitt, a ''cold, ruthless act of terrorism'' before sentencing Jose Dionisio Suarez Esquivel to the maximum prison term allowed under a plea agreement.

Suarez, a member of the Cuban National Movement, was described by his attorney as an anti-Castro Cuban.

''In all his political zeal, he turned to one of the few countries in the world'' that could have given him freedom, Robinson said referring to Suarez's move to the United States. But ''he chose not to be as free'' as everyone else, the judge said.

Letelier and Moffitt were killed here Sept. 21, 1976, when a bomb exploded beneath the former ambassador's car. Letelier, Chile's ambassador to Washington from 1971 to 1973, was an outspoken critic of the right-wing military government of Gen. Augusto Pinochet.

Suarez, who had been a fugitive for a dozen years, in September admitted giving a blasting cap to former Chilean intelligence agent Michael V. Townley, who allegedly masterminded the assassination.

In exchange for Suarez's plea, authorities agreed to drop charges of concealing a fugitive against his wife, Elizabeth Gongora Suarez, officials said.

Suarez, 51, will be eligible for parole after serving nine years.

Two other people have been convicted in connection with the bombing. Townley was sentenced to 10 years, while former Chilean army Maj. Armando Fernandez Larios received seven years.

Fernandez defected from Chile in 1987 and pleaded guilty to an accessory after the fact. He implicated two of his superiors and Pinochet in the coverup of the Chilean military's involvement in the crime.

The two Chilean military officials have been charged with participating in the murder, but the Chilean government has refused to extradite them.