At least four people have been killed after a pedestrian bridge collapsed over a six-lane highway at a Miami college, trapping people and cars underneath.

The newly-installed campus bridge had been declared an "architectural feat", and just days before its collapse the engineering team behind it congratulated its workers "a job well done".

In a now deleted tweet from March 12, structural testing service BDI said it was "thrilled" to have performed monitoring on the bridge, and congratuled its builders.

A Miami-Dade Fire Rescue firefighter looks into a heavily-damaged pickup truck. Photo / AP
A Miami-Dade Fire Rescue firefighter looks into a heavily-damaged pickup truck. Photo / AP

The bridge collapsed on Thursday afternoon local time, sparking a frantic rescue operation.

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Miami-Dade Fire Rescue Chief Dave Downey said the victims have yet to be identified as authorities were working to remove the debris piece by piece.

"We have located victims, but we're unable to identify them or have any confirmation," he said.

Emergency personnel worked frantically to find people in the rubble, with ambulances, fire trucks and police cars attending the scene after the sudden collapse of the 950-ton bridge.

At least eight vehicles under massive slabs of concrete and steel with Sweetwater Mayor Orlando Lopez saying "we have a national tragedy on our hands."

According to the Miami Herald, police fear there could be up to six people dead but have not been able to confirm that figure.

Officials from the Kendall Regional Medical Center said the hospital has received 10 patients.

Two of these were described as being in an "extremely critical" condition, while the other eight are in a stable condition.

Of those, two were in "extremely critical" condition and the other eight were stable with injuries such as broken bones, bruises and abrasions.

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Medical staff told local ABC affiliate 10 News there was an "intense fight" to keep those pulled from the rubble alive.

Dr Mark McKenney, the hospital's director of general surgery said of the two more serious cases, one arrived in cardiac arrest but was revived. The other had a serious brain injury. It's believed the patient with a head injury was now in a coma — that person and the cardiac arrest patient were listed as "very critical".

The ages of the all the injured ranged from 20 to 50.

A hospital source told 10 News the scene at the hospital was chaotic as patients came in "back to back"

Officials on the ground have described desperate scenes as emergency workers drill holes into the debris in the search for survivors.

Dr McKenney said it was the most patients to come in at once they had ever seen. Hospital staff were also waiting in preparation for more victims to be brought in, once they are rescued from the rubble.

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Miami-Dade Fire Rescue Division Chief Paul Estopinan said rescuers are searching the rubble for people who may still be trapped underneath but had no idea know how many they would find.

Miami Herald reporter Monique O. Madan tweeted police had confirmed there were mass casualties.

She also tweeted a video of the aftermath of the tragedy with people being warned to stay amid fears the rest of the bridge could collapse.

'COLLAPSED OUT OF NOWHERE'

The collapse came as it emerged the bridge was supposed to withstand a Category 5 hurricane.

Witnesses said there was a worker on top of the bridge when it collapsed just before 2pm on Thursday local time.

"The bridge at FIU just collapsed out of nowhere," said Gabriella Collazo in a video she shared on Twitter. "Workers are hurt, they're pulling workers out. There's definitely people stuck under ... this is crazy, god bless everyone involved."

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One driver who was waiting at the lights told MSNBC: "I was three seconds away from being under the bridge.

Police closed the highway after the collapse and local news network WSVN reported that TV news helicopters were ordered to back off so rescuers could listen for sounds from survivors.

Medical students also raced to the scene to offer help.

Sweetwater police Detective Juan Llera was in a nearby meeting when the bridge collapsed.

"I heard a 'boom' like a bomb had exploded," he said. "At first I thought it was a terrorist attack."

He said he saw three construction workers who had been injured. One had a head injury and was passing in and out of consciousness, another one had a leg injury leg and the third was lying on the street unconscious. He started performing CPR on him.

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"We were able to keep him alive to send him to the hospital," he said.

Witness Isabella Carrasco said she felt lucky to be alive. She told CNN she heard "a loud bang" before turning and seeing the bridge on the road.

"The cars were completely crushed," she said. "You could see some of the car and just a lot of debris everywhere."

She said she saw one woman get out of a car that was "just nicked" and rescue personnel performing CPR on another person in the street. "Other than that, I didn't see anybody else."

The collapsed pedestrian bridge at Florida International University in the Miami area. Photo / AP
The collapsed pedestrian bridge at Florida International University in the Miami area. Photo / AP

Witness Tiona Page told ABC News in the US that the screams coming from the cars were "terrifying."

"As soon as I looked outside, I saw dust flying everywhere," she said. "I knew the bridge had collapsed."

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Lynell Collins, who was driving past when he saw the bridge come down, rushed to help rescue people in cars trapped under the bridge.

After the whole thing broke, I was freaking out," he told CNN. "I got out of my car and me and a few other people were sprinting over there. We started helping people whose cars were at least half crushed and whoever was easily saved.

"We started helping people whose cars were at least half crushed and whoever was easily saved. But we couldn't really go under any of the rubble because at the time, the bridge — the other half of the bridge is still kind of leaning upward right now. That was also looking like that would slip and fall."

He said he saw two trucks that were "completely crushed" and other vehicles on the other side of the bridge that he could not reach.

"Somebody we helped said his family was inside and we couldn't help get them out. I'm sorry. I'm still shaken right now."

Others described their shock as the bridge came down.

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US President Donald Trump thanked the workers on the scene of the tragedy and said his thoughts were with those affected.

'ENGINEERING FEAT'

The Miami Herald reported the collapsed bridge, which connected Florida International University to a student housing area, had only been installed on Saturday and was not yet open.

The bridge was intended to make crossing the busy road safer for students and was not expected to open until 2019.

The bridge was funded by an $11.4 million grant from the US Department of Transportation.

According to the university, the bridge is an "engineering feat" and was designed to withstand even the most severe hurricanes.

It also said the bridge was supposed to have a durability of 100 years and was constructed entirely of self-cleaning concrete.

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It also emerged the bridge had undergone recent structural testing just days ago.

In a now deleted tweet from March 12, structural testing service BDI said it was thrilled to have performed structural monitoring on the bridge and congratulated the team on a "job well done".

In a statement posted on Twitter, the company said it was saddened by the news of the collapse, adding "our hearts go to the individuals and families affected by the collapse."

In a statement posted to social media, Munilla Construction Management, one of the companies involved in the construction of the bridge, said it would conduct a full investigation to see what went wrong.

"Our family's thoughts and prayers go out to everyone affected by this terrible tragedy. The new UniversityCity Bridge, which was under construction, experienced a catastrophic collapse causing injuries and loss of life," it said.

"MCM is a family business and we are all devastated and doing everything we can to assist. "We will conduct a full investigation to determine exactly what went wrong and will co-operate with investigators on scene in every way."

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FIGG Engineering, the company which designed the bridge, said it would fully co-operate with every appropriate authority in reviewing what happened, Fox News reported.

It also said in their "40-year history, nothing like this has ever happened before".

Meanwhile, an engineering professor said he believed the decision to use what the builders called an "innovative installation" for the bridge was a risky one.

Robert Bea, of the University of California, Berkeley, told the Associated Press it was too early to know exactly what happened.

However, he raised questions over the builders' decision to try a "rapid span installation" where the span was prefabricated, then swung into place before its central support tower was built.

The section was put into place across a busy eight-lane road on Saturday.

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Professor Bea said trying something new that crossed a highway was a gamble.

The main part of the 53-metre span was assembled by the side of the road while support towers were built at either end.

Florida Governor Rick Scott tweeted he was on his way to be briefed on the tragedy and

offered Miami-Dade County "every state resource to assist with the response to this tragedy".

Florida International University tweeted that it was "shocked and saddened about the tragic events unfolding", advising people to avoid the area.

The university had praised the "first-of-its-kind" bridge as it "swung into place" on Saturday, tweeting: "This project accomplishes our mission beautifully."

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— with Andrew Koubaridis and the Associated Press