Rescue teams frantically dug through rubble on Thursday for at least 22 people missing after three buildings collapsed in the historic heart of Rio, killing at least four people, officials said.

Rio Mayor Eduardo Paes offered the new toll at a late press conference, explaining that the figures had been adjusted - the earlier toll stood at five dead and 16 missing - after rescue personnel had compiled their findings.

The buildings collapsed late on Wednesday near the municipal theater on the city's Cinelandia square, in a historic district bustling by day but nearly deserted at night.

The deadly incident raised fresh questions about the state of Brazil's infrastructure as Latin America's dominant economic power prepares to host the 2014 World Cup and 2016 Summer Olympics.

Brazil had its share of disasters in recent months, including explosions caused by gas leaks, and fatal accidents in poorly maintained amusement parks or in packed streetcars with faulty brakes.

The three buildings - of 20, 10 and four floors - collapsed nearly simultaneously for as yet unknown reasons, though authorities said they did not believe an explosion had occurred as witnesses initially reported.

"The possibilities of an explosion are near zero," said Paes, adding that the tallest building, which was undergoing renovations, probably collapsed due to structural weakness and brought down the other two.

"We remained focused on what is most relevant, which are the human lives," Paes told reporters.

Guided by sniffer dogs, rescuers clawed away at the mountain of rubble searching for survivors.

"We believe that it's possible to find people in spaces where there is air," said Julio Cesar Mafia, a military police officer. "We are taking great care to avoid further collapses."

Paes - who declared three days of mourning from Friday - said some 1,500 tons of debris was removed and taken to a 'safe location' to be studied to determine why the buildings collapsed.

Civil defense teams inspected nearby buildings, including a nine-story structure on an adjacent street that was evacuated as a precaution.

Police spokesman Rodrigo Pimentel told reporters that 'illegal' work had been taking place in the tallest high-rise, which housed several law offices.

"This may have affected the structure of the building. Construction material was also stored inside and this could have caused a breakup of the structure," he added.

President Dilma Rousseff, on a visit to the southern city of Porto Alegre, expressed her deep sorrow over the tragedy and said she hoped survivors would be found.

"My heart goes out" to the victims, said Rousseff, who added that she was closely monitoring rescue efforts.

Rousseff was in Porto Alegre to attend the World Social Forum, which brings together thousands of militants seeking an alternative to capitalism.

Relatives of the missing meanwhile anxiously waited for news.

Vera de Anjo Freitas desperately sought information about her cousin Moises Morais Silva, who was with her in the street soon before the collapse.

"I felt something like a rock falling on my head and I ran away," the 57-year-old maid said.

Her cousin and his four work colleagues "were right in front of the building," she said, holding back her tears. "I don't think they are alive."

Brazilian authorities are racing to build or renovate 12 stadiums around the country in time for the 2014 World Cup, one of the world's premier sporting events.

Last month football's ruling body FIFA warned Brazil about delays in the progress of construction projects expected to be ready for the four-yearly extravaganza.

The Getulio Vargas Foundation and consultancy Ernst & Young have said Brazil needs more than $11 billion in investment to fix roads, boost hotel capacity, reinforce security and develop its telecom network ahead of the World Cup.

Sergio Magalhaes, president of Rio's Architecture Institute, said modest investments in infrastructure projects in both Rio and around Brazil were not commensurate with soaring needs and a growing population.