Six presumed dead in Baltimore bridge collapse

Authorities have suspended their search for six people missing after a packed cargo ship slammed into a Baltimore bridge, causing it to collapse and blocking one of the busiest US commercial harbours.

"Based on the length of time that we've gone in this search, the extensive search efforts that we put into it, the water temperature... 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 a press conference on Tuesday.

All six people were members of a nighttime construction crew who were repairing potholes on the Francis Scott Key Bridge when disaster struck not long after midnight.

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

"We're going away from the search and rescue portion to a recovery operation," said Roland Butler, Maryland's secretary of state police. He said the temperatures and currents were making it difficult for divers to continue working underwater, but that boats would continue patrolling overnight.

Video footage showed the packed container ship slamming into one of the bridge's supports, causing the 1977-built steel structure to collapse like a deck of cards.

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

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.

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

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.