At least seven people are dead after part of a ferry dock collapsed in the US state of Georgia, according to authorities.

Crowds had gathered at Sapelo Island for an autumn celebration by the island's tiny Gullah-Geechee community of Black slave descendants.

Victims were taken to hospitals, and crews from the US Coast Guard, the McIntosh County Fire Department, the Georgia Department of Natural Resources and others were searching the water, according to Natural Resources spokesperson Tyler Jones. 

The agency operates the dock and ferry boats that transport people between the island and the mainland.

Mr Jones said a gangway at the dock collapsed, dropping people into the water below.

"There have been seven fatalities confirmed," Mr Jones said. 

"There have been multiple people transported to area hospitals, and we are continuing to search the water for individuals."

Helicopters and boats with side-scanning sonar were used in the search, according to a Department of Natural Resources statement.

Among the dead was a chaplain for the state agency, Mr Jones said.

He said he did not know what caused the gangway to collapse, but officials believed there were at least 20 people on it at the time. 

The gangway connected an outer dock where people board the ferry to another dock onshore.

Sapelo Island is about 100 kilometres south of Savannah, reachable from the mainland by boat.

The deadly collapse happened as island residents, family members and tourists gathered for Cultural Day, an annual event spotlighting the island's tiny community of Hogg Hummock, home to a few dozen Black residents. 

The community of dirt roads and modest homes was founded after the Civil War by former slaves from the cotton plantation of Thomas Spalding.

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. 

Scholars say their separation from the mainland caused residents to retain much of their African heritage, from their unique dialect to skills and crafts such as cast-net fishing and weaving baskets.

In 1996, Hogg Hummock, also known as Hog Hammock, was put on the National Register of Historic Places, the official list of the United States' treasured historic sites.

But the community's population has been shrinking for decades, and some families have sold their land to outsiders who built vacation homes.

Tax increases and zoning changes by the local government in McIntosh County have been met by protests and lawsuits by Hogg Hummock residents and landowners. 

They have been battling for the past year to undo zoning changes approved by county commissioners in September 2023 that doubled the size of homes allowed in Hogg Hummock.

Residents say they fear larger homes will lead to tax increases that could force them to sell land their families have held for generations.

AP