Video has been released which shows the moment a bridge collapsed onto a major road in Miami, killing six people and leaving others trapped and injured under massive slabs of concrete and steel.

The CCTV footage posted on Twitter shows the bridge, which was still being built, folding in on itself at the centre, sending plumes of smashed concrete into the air.

The death toll rose early on Friday from four to six people after emergency teams worked through the night in a desperate effort to find survivors among the debris. Crews used sniffer dogs, search cameras and sensitive listening devices to hunt for people still alive.

The video shows several cars narrowly missing being crushed but others are caught underneath the structure as it collapses onto the seven-lane route. Eight vehicles were crushed, according to Florida Highway Patrol officials.

At least 10 people were taken to hospital immediately after the disaster happened. Of those, two were in “extremely critical” condition, doctors said.

Miami-Dade police chief Juan Perez said hopes were dwindling as the hours passed. “We know that there's going to be a negative outcome at the end of the day,” he said.

The 950-ton span was installed only on Saturday to great fanfare, and work was still being carried out on the unfinished structure.

An "accelerated" construction method was supposed to reduce risks to workers and pedestrians and minimise traffic disruption.

FIU President Mark Rosenberg says a thorough investigation will commence into Florida bridge collapse

National Transportation Safety Board chairman Robert Sumwalt III said a team of specialists were launching an investigation into how the collapse happened.

The remaining structure still poses some danger. Miami-Dade County Fire Chief Dave Downey said: “We have to remove some of this piece by piece. It's very unstable.”

Jacob Miller, a witness, described what happened: “I saw there were multiple cars crushed under the bridge. It was just terrible. I saw some people stopping their cars, trying to get out, trying to assess the situation to see if there is anything they could do to help.”

Florida Governor Rick Scott said an investigation would get to the bottom of “why this happened and what happened” He said that if anyone did anything wrong, “we will hold them accountable”.

US Senator Marco Rubio said the public and the families of the dead and injured deserved to know “what went wrong”.

Mr Rubio, who is an adjunct professor at the school, said the pedestrian bridge was intended to be an innovative and “one-of-a-kind engineering design”.

The US$14.2m bridge crossed over a busy road that divided a Florida International University campus from the city of Sweetwater.

The school has long been interested in this kind of bridge design; in 2010, it opened “the ABC (accelerated bridge construction) Center” to help construction professionals.

Critics said the bridge should not have been allowed to open yet.

The president of Florida International University paid his condolences on Twitter, saying the bridge collapse in which four people died “stuns us, it saddens us”.

“It's exactly the opposite of what we had intended and we want to express our deepest condolences to the family and loved ones of those who have been affected,” Mark Rosenberg said.

The project was a collaboration between MCM Construction, a Miami-based contractor, and Figg Bridge Design, based in Tallahassee. Figg said the company was “stunned” by the collapse and promised to cooperate with investigations.

“In our 40-year history, nothing like this has ever happened before,” the company's statement said. “Our entire team mourns the loss of life and injuries associated with this devastating tragedy, and our prayers go out to all involved.”