In a landmark ruling, it said abduction cases where the victim or remains are not found are not covered by an amnesty law issued by Gen Pinochet.

The court also upheld the convictions of five former aides to Manuel Contreras over the 1975 disappearance.

Human rights groups said the move was a "great victory" for accountability.

"Today's ruling gives full backing to efforts by the lower courts to hold accountable those responsible for grave human rights violations under military rule," said Jose Miguel Vivanco of Human Rights Watch.

"It is a great victory for the victims' families and their lawyers, who have battled for years to bring this about."

Previous conviction

Miguel Angel Sandoval, a member of the Movement of the Revolutionary Left, vanished after he was arrested by secret police agents in January 1975.

His body has never been found.

However, the Supreme Court reduced Contreras' prison sentence from 15 to 12 years, for masterminding the disappearance.

Correspondents say Wednesday's decision sets a precedent for many other unresolved disappearances that took place under Gen Pinochet's campaign against dissidents.

Thousands of supporters of the previous government were killed, tortured or forced into exile during his 1973-1990 military rule.

In 1978, Gen Pinochet issued an amnesty that covered human rights crimes committed during the first five years of his regime.

Contreras' lawyer said he should have been granted amnesty.

The founder of Gen Pinochet's much-feared secret police had already been convicted and served seven years for the killing of a former Chilean foreign minister.

Contreras set up the unit that played a key role in the early years of repression of the military government.