Authorities on Tuesday 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 harbors.

"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 as night fell.

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.

"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 then collapse like a deck of cards.

Maryland Governor Wes Moore said earlier that quick thinking by authorities once the ship issued a Mayday call allowed them to stop vehicles from driving onto the bridge.

"These people are heroes. They saved lives last night," he told a press conference.

The FBI and other agencies stressed that there was no known connection to terrorism.

"The preliminary investigation points to an accident," Moore said.

The governor, a rising star in the Democratic party, also said there was no indication that the bridge was structurally at fault, noting that it "was actually fully up to code."

"The crew that was out there working was basically repairing potholes, just so you understand that had nothing to do with a structural issue at all," Paul Wiedefeld, the Maryland transportation chief, said.

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

Two of the missing workers were from Guatemala, the country's foreign ministry said.

Attempt to drop anchors

Across the middle of the Patapsco River, steel girders protruded from the water, with mangled metal draped over the deck of the ship.

"This is something we've been looking at our whole lives," said local teacher Tiffany Wengert, 30.

"And all of the sudden... it's just gone."

Details emerged on how the crew tried to avert disaster after their 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 and collided," said the maritime authority for Singapore, where the Dali is flagged.

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

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.

Butler, with Maryland state police, said divers on Wednesday morning will "begin a more detailed search to do our very best to recover those six missing people."

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 harbor. However, the tangled steel barrier now lying half-submerged across the harbor entrance blocks almost all maritime traffic.

Early 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," Butler said.

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.

"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.

Baltimore Mayor Brandon Scott described the bridge as an "unthinkable tragedy... like something out of an action movie."

"We have to be thinking about the families and people impacted," he said.