Engineers have started work on the process of cutting and lifting the first section of twisted steel from a bridge which collapsed in Maryland after a cargo ship crashed into one of its main supports.

The Francis Scott Key Bridge crumpled into the Patapsco River on Tuesday after the incident.

US Coast Guard officials confirmed work has started to remove a section of the toppled structure on Saturday and sparks could be seen flying from a section of bent and crumpled steel.

Crews are carefully measuring and cutting the steel from the broken bridge before attaching straps so it can be lifted onto a barge and floated away, Coast Guard Rear Admiral Shannon Gilreath said.

Seven floating cranes — including a massive one capable of lifting 1,000 tons — 10 tugboats, nine barges, eight salvage vessels and five Coast Guard boats are on site in the water south-east of Baltimore.

Each movement affects what happens next and ultimately how long it will take to remove all the debris and reopen the ship channel, and the blocked Port of Baltimore, Maryland governor Wes Moore said.

He said: “I cannot stress enough how important today and the first movement of this bridge and of the wreckage is. This is going to be a remarkably complicated process.”

Undeterred by the chilly morning weather, long-time Baltimore resident Randy Lichtenberg and others took smartphone photos of the broken pieces of the bridge.

From a spot on the river called Sparrows Point, Mr Lichtenberg said: “I wouldn’t want to be in that water. It’s got to be cold. It’s a tough job.”

The shock of waking up on Tuesday morning to video of what he called an iconic part of the Baltimore skyline falling into the water has given way to sadness.

“It never hits you that quickly. It’s just unbelievable,” Mr Lichtenberg said.

One of the first goals for crews on the water is to get a smaller auxiliary ship channel open so tugboats and other small barges can move freely.

Crews also want to stabilise the site so divers can continue a search for four missing workers who are presumed dead.

Two workers were rescued from the water in the hours following the bridge collapse and the bodies of two more were recovered from a pick-up truck that fell and was submerged in the river.

They had been filling potholes on the bridge and while police were able to stop vehicle traffic after the ship called in a mayday, they could not get to the construction crew, who were from Mexico, Guatemala, Honduras and El Salvador.

The crew of the cargo ship Dali, which is managed by Synergy Marine Group, remains on board with the debris from the bridge around it. They are safe and are being interviewed.

They are keeping the ship running as they will be needed to get it out of the channel once more debris has been removed.

The vessel is owned by Grace Ocean Private Ltd. and was chartered by Danish shipping giant Maersk.

The collision and collapse appeared to be an accident that came after the ship lost power. Federal and state investigators are still trying to determine why.