At least seven people have died after part of a ferry dock collapsed in Georgia's Sapelo Island on Saturday, local authorities say.

Georgia's department of natural resources, which operates the dock, said at least 20 people plunged into the water when the gangway collapsed.

The incident happened at approximately 16:30 local time (20:30 GMT) at the Marsh Landing Dock as crowds gathered for a cultural celebration.

Multiple people have been taken to hospital and search and rescue operations are under way.

US President Joe Biden said he and his wife Jill mourn the lives lost and "pray for the injured and anyone still missing".

It is not yet known what caused the collapse of the walkway, which connected an outer dock where people board the ferry to another dock onshore, according to local reports.

Brian Kemp, the governor of Georgia, said he and his family were "heartbroken" by the tragedy and asked for prayers "for those lost, for those still in harm's way, and for their families".

The governor has sent "state resources to aid in search, rescue, & recovery", Georgia representative Buddy Carter said in a post on X.

Biden said his team "stand ready to provide any and all assistance that would be helpful to the community".

Local authorities said the gangway has been secured and the incident is under investigation.

Tyler Jones, a spokesperson for Georgia's National Resources Department, said there was "no collision," according to the Associated Press agency.

“The thing just collapsed," he said. "We don’t know why.”

Pastor Jerald Thomas of the nearby Elm Grove Church went to the scene to help.

“They came together quickly, and they began to send the necessary things that was needed in order to make the people stay while they were going through this tragic time,” he told local outlet WTOC.

People had gathered Saturday to celebrate the island's community of Hogg Hummock, which is home to a few dozen black residents.

Hogg Hummock was founded by newly-freed former enslaved people from plantations in coastal Georgia who settled on Sapelo Island following the US Civil War, the Georgia Trust for Historic Preservation said on its website.

Saturday's event "should have been a joyous celebration of Gullah-Geechee culture and history instead turned into tragedy and devastation", President Biden said in his statement.

Small communities descended from enslaved island populations in the South, known as Gullah, or Geechee in Georgia, are scattered along the coast from North Carolina to Florida.

Sapelo Island is reachable from the mainland by boat.