The Coast Guard said in a statement that crew members launched a life raft and four of the 21 mariners aboard left the Sincerity Ace wearing life jackets. The agency could not confirm if they reached the life raft. The remaining 17 crew members stayed on board to fight the fire.
The Coast Guard said other commercial vessels were helping with the rescue 1,800 nautical miles (3,333 kilometers) northwest of Hawaii. The statement doesn't say what caused the fire on the 650-foot (198-meter) ship.
"The distance involved here requires any and all available support, we thank the crews of the commercial vessels for volunteering to assist and have Coast Guard air support en route," Petty Officer 1st Class Les Elliott said in the statement.
One commercial vessel had arrived and was assessing possible rescue options, the agency said. Three additional commercial vessels and Coast Guard airplanes were heading there.
"We are thankful no injuries have been reported but are cognizant a lot is happening, and that may change at any time," Elliott said.
The Coast Guard said the U.S. Navy will help with search efforts.
A Coast Guard spokesman in Honolulu said it was unknown how many cars the ship is carrying.