MEXICO CITY (AP) _ Rescue workers said they shouted through mounds of rubble late this morning and heard the faint voice of a 9-year-old boy buried for 15 days under a collapsed apartment building, demolished by last month's devastating earthquake.

Jorge Negrete, a spokesman for the workers, said rescuers asked the boy if he was there and the child answered back: ''Si.'' Negrete said they spoke to the boy at 10:45 a.m. (12:45 EDT). He said the workers did not try to talk him further because they didn't want to exhaust him.

Negrete said the brief talk with the boy came after the discovery of a cistern in the debris. He said it was trickling water to where the child was believed trapped.

''There are indications of life,'' said Carlos Malbran, an Argentine engineer who helped organize the rescue effort.

Mexico City police Chief Ramon Mota Sanchez, who spent most of the night at the site, said workers first made contact with the child by knocking at 4:10 a.m.

Sanchez said rescue workers, including a team from the Red Cross, had dug five tunnels and were concentrating on two of them.

Three rescue squads, wearing masks, hard hats and tape on their hands to protect their hands while they grovel through the dirt, have been working since Sunday to reach the earthquake victim. The workers have identified him as Luis Ramon Navarrete.

The boy was at home in his family's apartment with his grandfather, Luis Maldonado, 57, when the three-story building collapsed in the Sept. 19 earthquake, which measured 8.1 on the Richter scale. The workers at the site say it is believed the boy's grandfather did not survive.

A second quake, measuring 7.5 on the Richter scale, further crumbled buildings when it rumbled through the city one day after the first temblor.

Jose Negrete, a spokesman for the rescuers, had said ''little knocks'' also reached them at 8 p.m. and 10:20 p.m. Thursday. He said the contact was ''nothing more than signals.''

Workers had expressed fear Wednesday night that the boy was dead, after his tapping grew gradually fainter then stopped.

The boy's father, Mauricio Alberto Navarrete, said rescuers apparently were nearing the youth. ''In the begining we had to cup our hands to shout'' to the boy, he said. ''Now it is not necessary to shout.'' But neither he nor the rescuers could estimate how far they were from the youth.

Navarrete said rescuers had asked the boy to tap in response to questions.

''We asked if he was a child or an adult. Two knocks would be an adult, one knock would be a child and he responded that he was a child,'' the father said.

Rescue workers took reporters on a brief tour Thursday afternoon of the 20- square-foot courtyard area where they have been digging. They said the operation was proceeding so slowly because they had to hollow out a circuitous path to avoid dislodging rubble and hurting the boy.

The patio was covered with tons of dirt, concrete slabs and steel beams from the collapsed building next door.

Eight army generals arrived for what appeared to be an inspection tour of the building's courtyard Thursday. They were seen conferring with the rescue workers and left without comment.

Amid the rubble sat a 21/2 -foot-tall stuffed yellow lion, a gift for the trapped boy.

In his second speech to the nation since the earthquake, President Miguel de la Madrid said Thursday night the capital must start rebuilding, although workers should continue searching for any more survivors and recovering bodies.

He said the city still is suffering from a severe shortage of water, and also has problems with reduced public transportation, teetering buildings, a lack of food and housing for the homeless and the failure of long-distance telephone service.

''Although we continue to search intensely for survivors, the hope of having success is less each day,'' he said. ''We still have many dead to recover and we must try to give them proper burial.''

The government death toll has stood at 4,600 since last weekend and the president gave no new figures.

De la Madrid said the number of people seeking to adopt children orphaned by the earthquake exceeded the number of orphans.

''I cannot hide from you that the damage we have suffered from the earthquake complicates the already serious problems that we had in managing the economy, both in domestic aspects and in international relations,'' he said.

A total of 2,831 buildings were damaged in the earthquake, the television news program 24 Horas reported Thursday night.