(CNN) The US Coast Guard was working Monday to rescue four South Korean crew members aboard a cargo ship that tipped onto its side after it began listing heavily off the Georgia coast, the South Korean Foreign Ministry said.

Rescuers are not sure where on the vessel the crew members are located, and complicating the search are the size of the ship and the fact it has watertight doors, numerous compartments and no power, Coast Guard Petty Officer 1st Class Luke Clayton said.

Video from the scene showed a red Coast Guard helicopter landing on the ship's side mid-morning Monday.

The 656-foot Golden Ray's crew informed authorities that the vessel was listing about 2 a.m. Sunday. By evening, it was laid over on its side in St. Simons Sound as authorities worked to stabilize the ship so they could resume rescue operations, which were stalled due to a fire on the ship, Capt. John Reed, commander of the Coast Guard Sector Charleston, said at a news conference.

Crews from the Coast Guard and other agencies responded and rescued 20 of the 24 people on board -- 23 crew members and one pilot -- before a fire forced them to stop, he said. The Coast Guard said the ship is carrying vehicles.

Read More