Dozens of people were injured, some of them seriously, in an explosion at a sugar refinery in the U.S. state of Georgia on Thursday night, local authorities said, adding that no fatalities had been reported yet.

Six people were unaccounted for and firefighters were attempting to put out the blaze apparently started by a blast in a bagging room at the Imperial Sugar company plant in Port Wentworth, a suburb of Savannah, said police Sgt. Mike Wilson.

Around 38 people were taken to hospital but the number of people with minor injuries was higher, he said, adding that the cause of the early evening blast was not known.

"We are still in a search and rescue operation. This is still a fluid and active scene. We still have firefighters actively fighting the blaze at this point," Wilson said at around midnight local time (5 a.m. British time).

Coast Guard vessels equipped with water pumps were also helping the effort, hours after fire erupted at the plant along the banks of the Savannah River. "Portions of it (the plant) have collapsed," he said.

Jay David Goldstein, chief of emergency medicine at Memorial University Medical Centre in Savannah, described the incident as: "serious, pretty significant" and said many of the plant workers injured were in critical condition.

The injuries included "significant body burns, and you also have to worry about blast effect from the explosion, which might throw them 20 feet (6 metres), and then you have head or bone injuries," he said.

A Memorial hospital statement said 32 patients were taken to Memorial and five patients were also brought to St Joseph's Candler hospital in Savannah, Candler spokeswoman Betsy Yates said.

The explosion happened at around 7:20 p.m./12:20 a.m. British time and emergency responders headed to the scene from throughout the area, the Savannah Morning News newspaper said. Doctors also flew in from Augusta, Georgia, to provide assistance.

Imperial Sugar is the largest processor and refiner of sugar in the United States and is based in Texas. It includes the Imperial Sugar brand in that state and the Dixie Crystals brand in the south east, according to a company Web site.