US emergency teams are searching for bodies a day after a bridge over the Mississippi river collapsed, killing a number of people.

At least four people are confirmed dead with up to 30 missing. US officials are not linking the collapse to terrorism.

Some 50 vehicles were hurled into the water when part of the I-35W bridge in Minneapolis, Minnesota, gave way during the rush hour on Wednesday evening.

Rescue officials said they expected the death toll to rise.

Minneapolis police chief Tim Dolan said people were still trapped inside vehicles and the recovery effort would take at least three days.

"We're dealing with the Mississippi river, we're dealing with currents and we're going to have to do it slowly and safely," Mr Dolan said.

Investigators are treating the area as a crime scene until they determine a cause for the collapse.

US President George W Bush said the federal government must "respond robustly" to help the people of Minnesota recover and rebuild the bridge as quickly as possible.

The federal Department of Transportation has offered an initial $5m  to help with clean-up efforts and transport issues.

Minnesota Governor Tim Pawlenty said the collapse was a "catastrophe of historic proportions".

The 40-year-old I-35W highway bridge was being repaired at the time of the collapse.

Mr Pawlenty ordered an immediate inspection of all bridges in the state with similar designs. He said the bridge was last inspected in 2006 and no significant structural problems were found.

But in 2005, the bridge was one of thousands across the US rated as "structurally deficient" on the federal National Bridge Inventory database.

The bridge rated 50 on a scale of 100 for structural stability in that study, White House press secretary Tony Snow said.

Traffic had been "bumper to bumper" at the time of the incident as only one lane was open in either direction on the eight-lane road because of construction work on the arched bridge.

Tons of concrete crashed 64 feet (20m) into the water as the structure crumbled.

Injured people waited for rescue as vehicles dangled off the crumpled concrete and smoke billowed into the sky from the fiery wreckage of a lorry.

Some people managed to swim to safety.

"The bridge started falling, cars were flying everywhere and I saw the water coming up," Catherine Yankelevich, whose car ended up in the water, told AFP news agency.

"The water was coming up pretty fast, so I rolled the window down... It seemed like a movie, and it was pretty scary."

At least 50 children were led to safety with only minor injuries from a school bus that clung to the side of a collapsed section over the river.

Huge roar

Police said officials had accounted for all but one of the construction workers who had been on the bridge at the time of the collapse.

Eyewitnesses said they heard a huge roar as the structure fell into the river.

"What I first heard was a giant rumbling sound from my apartment," Mark Lacroix told the BBC.

"I'm 20 storeys up, just a stone's throw from the bridge itself, and I thought, maybe my own building was coming down, it was an amazing noise. And I look out of the window and I see just the final moments of it collapsing into the river.

"It really took quite a while for me to realize that this was not a planned explosion... to realize it was a tragedy."

PHOTO CAPTION

The bridge over the Mississippi river after collapsing