Search-and-rescue workers were looking for victims amid the rubble in Rio de Janeiro's historic city centre Thursday after a 20-storey building collapsed onto two other structures.

As many as 20 people may be missing with three buildings involved, reported the CBC's Latin America correspondent, Connie Watson.

Witnesses reported a strong smell of gas in the area, but it wasn't yet known what caused the buildings to partially collapse.

Firefighters and rescue teams were combing through the debris overnight, as officials put together a list of the missing. Local media reported at least 20 people were believed to be unaccounted for early Thursday.

There have been no confirmed deaths, despite earlier reports that several people had died. At least five people were taken to hospital.

The accident could have been much worse, Watson reported. Most of the buildings' tenants were businesses that had were closed for the day when the tower tumbled onto two smaller buildings.

"At first, I thought someone was just throwing rubble," one witness said, speaking Portuguese.

"Then all of a sudden, the building collapsed. It came down completely. It looked like Sept. 11 here."

Renovation jobs underway in building

Other witnesses in Rio de Janeiro's old city centre reported a loud explosion and a strong smell of gas just before the collapse.

The city's mayor is asking investigators to look into structural problems. The central part of Rio is home to a large number of older buildings of questionable strength and deteriorating infrastructure that are badly in need of repair.

At least two renovation jobs were underway in the 20-storey building. It's not known whether the crews had permits for their work.

Officials said their priority is to locate the 20 people believed to be missing, in case any are trapped in the rubble that has blocked at least one city street.

Rio is scheduled to host soccer's World Cup in 2014 and the Summer Olympics in 2016. Failures of infrastructure such as Wednesday's building collapse are raising concerns about whether the city is ready for the thousands of people who will descend on it for those international events.