(Updates death toll, quote)

By Matthew Bigg

ATLANTA, Feb 8 (Reuters) - Four people died, four others were missing and more than three dozen were injured in an explosion and fire at a sugar refinery in the U.S. state of Georgia, authorities said on Friday.

Portions of the riverfront plant collapsed in Thursday's blast, which may have happened in a sugar storage silo or bagging room at the Imperial Sugar Co (IPSU.O: Quote, Profile, Research) plant in Port Wentworth, a suburb of Savannah.

"It was a very large explosion that totally destroyed the three-story building where it took place," Georgia Fire Commissioner John Oxendine told Reuters. "The fire then spread through to the river ... The explosion just totally collapsed one floor on top of the other."

Imperial Sugar chief executive John Sheptor suggested that volatile sugar dust may have contributed to the blast, police said.

The reported death toll in the massive blast fluctuated throughout the day as rescue crews searched the flooded, fire-ravaged factory and piles of rubble. Police Sgt. Mike Wilson told Reuters that four bodies had been found and four people were still missing.

"We've been saying this is fluid and until we put hands on every single person we're not going to count anybody out," he said. "What we have unaccounted for is four."

Around 100 people were in the plant along the banks of the Savannah River when the blast ripped through at about 7:20 p.m. (0020 GMT), police said.