Authorities in Baltimore were set to focus on expanding recovery efforts Wednesday after a cargo ship slammed into a major bridge, causing it to collapse and leaving six people presumed dead.

All six were members of a construction crew repairing potholes on the Francis Scott Key Bridge in the US East Coast city when the structure was sent careening into the Patapsco River at around 1:30 am (0530 GMT).

A desperate search in near-freezing conditions managed to pull two people from the choppy waters, but after nearly 16 hours, officials said they had given up hope of rescuing the others.

"At this point we do not believe that we're going to find any of these individuals still alive," US Coast Guard Rear Admiral Shannon Gilreath told reporters as night fell on Tuesday.

He added that responders were transitioning "to a different phase."

Speaking at the same press conference, Maryland Secretary of State Police Roland Butler said the focus would shift to a "recovery operation" by Wednesday morning.

"We're hoping to put divers in the water and begin a more detailed search to do our very best to recover those six missing people," he said.

After the search effort was called off, Mayor Brandon Scott emphasized the need to help families of the victims "get the closure they deserve."

"My heart is with those families tonight and in the days ahead," he said in a statement.

One of the missing workers was father-of-three Miguel Luna, according to Casa, a nonprofit that serves immigrant communities.

Luna, from El Salvador, had left for work at 6:30 pm on Monday and never returned, Casa said.

His wife, Maria del Carmen Castellon, told Telemundo 44 that she was "devastated" by the wait for any information.

Two of the other missing workers were from Guatemala, the country's foreign ministry said, while local news outlet The Baltimore Banner reported that Mexican and Honduran nationals were also among the victims.

"My heart hurts with this situation," said Jesus Campos, a construction company employee who said he worked alongside the missing.

"They're human beings and they are my colleagues."

- Attempt to drop anchors -

Footage of the collision showed packed container ship the Dali slamming into one of the bridge's supports, causing the 47-year-old structure to collapse into one of the busiest US commercial harbors.

Officials stressed that there was no known connection to terrorism, and that the preliminary investigation indicated an accident.

Details emerged on how the crew tried to avert disaster after their 985-foot ship lost power and began careening toward the bridge.

"Just prior to the incident, the vessel, Dali, had experienced momentary loss of propulsion. As a result, it was unable to maintain the desired heading," said the maritime authority for Singapore, where the Dali is flagged.

The authority said the ship's management company, Synergy Marine Pte Ltd, reported the crew "dropped anchors" in a last-ditch attempt to hold it back.

The ship had passed two overseas inspections in 2023, the authority said Wednesday, adding that a fault monitor gauge was fixed in June.

Investigators from the authority and Singapore's Transport Safety Investigation Bureau were headed to Baltimore on Tuesday to assist the US Coast Guard.

US federal investigators expect recordings from the vessel to be critical to determine what happened, said Jennifer Homendy, head of the National Transportation Safety Board, which is overseeing the investigation.

"Certainly investigations are a priority, certainly environmental considerations are a priority, and so is traffic and getting cargo vessels in and out of the port of Baltimore," she said. "But right now it's about people, it's about families and addressing the needs of those impacted."

- Ninth-busiest US port -

The Francis Scott Key Bridge, named after the poet who penned the lyrics to the US national anthem, is an important link in the East Coast highway system, used by about 34,000 vehicles every day.

The Port of Baltimore is the ninth-busiest major US port in terms of both foreign cargo handled and foreign cargo value, and is directly responsible for more than 15,000 jobs, supporting almost 140,000 more.

US President Joe Biden called the collapse a "terrible accident," and pledged to get the port reopened and the bridge rebuilt.

There are other bridges and tunnels for drivers to cross the harbor. However, the tangled steel barrier now lying half-submerged across the harbor entrance blocks almost all maritime traffic.

"There is no question that this will be a major and protracted impact to supply chains," US Secretary of Transportation Pete Buttigieg warned, adding it was "too soon" to know when the port might reopen.