Six people are still missing after a major bridge in the US city of Baltimore near Washington collapsed on Tuesday after a ship crashed into it, the state Secretary of Transportation, Paul Wiedefeld, said at a press conference.
At least two people have been rescued following the collapse of the Francis Scott Key Bridge after a cargo ship hit the structure, the local fire authorities said. Baltimore, located in the state of Maryland, is about 62 kilometres north-east of the US capital.
One person is in critical condition in hospital, Fire Department chief James Wallace said.
Rescue efforts to find the missing in the river were ongoing. It was initially unclear how many people had been on the bridge at the time of the accident, so the number of missing could change.
Citing the US Coastguard and the fire department, the New York Times and CBS News reported that the first emergency calls were received at around 1:30 am (0530 GMT).
Maryland's governor, Wes Moore, has declared a state of emergency in the US state following the collapse.
Wiedefeld, Maryland's transportation secretary, called the accident a "catastrophic collapse" during a press conference, saying it was initially unlcear why the ship had rammed the bridge.
At the time of the collision, workers had been on the bridge carrying out concrete repairs, he said, but it was not known how many people had been on the structure when almost the entire above-water section of the 2.5-kilometre-long bridge collapsed.
Citing the fire department, CNN earlier reported that more than 20 people could have fallen into the cold water of the Patapsco River.
There were initially no reports of fatalities.
The rescue services have detected vehicles in the water using a sonar device, although it was not yet known how many, according to the fire department.
CCTV footage circulating on social media shows the ship ramming one of the support pillars, causing large parts of the bridge to collapse into the water, with several vehicles that were on the bridge at the time plunging into the river.
There was "absolutely no indication" that the ship had hit the bridge deliberately, Baltimore police chief Richard Worley said, adding that nothing suggested a terrorist motive.
The FBI has launched investigations into the incident.
The true extent of the overnight collapse only became apparent with sunrise, with the arched struts of the Francis Scott Key Bridge, a major link across the harbour of the metropolis in the north-east of the US, protruding from the water like ribs.
With people still suspected to be in the river, rescue workers are worried about the water temperature which CNN reported stood at some 9 degrees Celsius at the site of the collapse.
The human body can withstand such temperatures for between one and three hours before they could become fatal, it said.
The low temperatures as well as the current also posed a risk to rescue divers, fire department chief Wallace was quoted as saying by CNN.
The crew of the ship that caused the collapse was not harmed, the New York Times reported citing a statement by operator Synergy Group.
CBS News cited the coastguard as saying the vessel was the 290-metre Singapore-flagged cargo ship Dali apparently en route from Baltimore to Colombo in Sri Lanka.
The ship, which is operated by the charter company Synergy Group, was chartered by Maersk on a temporary basis, according to a statement from the Danish shipping giant on Tuesday. It is said to have been carrying freight from Maersk customers. According to the statement, no Maersk crew or personnel were on the ship. Maersk is the world's second-largest container shipping company after MSC.
It was unclear how many vehicles were on the four-lane motorway bridge at that time, but fire officials said "there was surely a large tractor-trailer" on it.
