Several Argentines believed to have been abducted and killed by security forces in the mid-1970s have turned up alive here recently, a major human rights group said this week.

The group, the Relatives of Disappeared People and Political Detainees, said in a letter to Argentine Roman Catholic bishops that the reappearances prove that military authorities have secret detention centers in which other prisoners could still be held.

Members of the same rights group charged in a petition to the courts yesterday that about 400 of the estimated 5,000 to 15,000 "disappeared persons" have been discovered in paupers' graves of a cemetery near the Army's main base.

"Let's face it, we live in a country in which everyone believes the disappeared are dead," said a spokesman for the group.

Public debate on the issue of the missing persons has amplified following the military defeat in the Falklands and pressures to accelerate a return to civilian rule. The letter to the bishops is the latest move in a running battle between some rights groups and the military over the alleged existence of secret detention centers as part of the campaign against leftist guerrillas. The government has denied such camps exist.

Asked about the group's claims, a spokesman for the Interior Ministry said, "We don't know anything more about it than we've read in the newspapers." Other human rights groups said they did not have specific information on the group's charges.

The relatives' group said it could not give details concerning the names of the persons who reappeared, their number or where they were held because of their physical condition. Also, "the people who left [imprisonment] are afraid, as are their families," said Catalina Silvia de Guagnini, a spokeswoman.