(CNN) -- A fire broke out in a passenger cabin as a ferry loaded with more than 1,000 people sailed across the North Sea from Newcastle, England, to the Netherlands on Saturday, a spokesman for the shipping company DFDS said.
Two Royal Air Force helicopters, as well as boats from the Humber Coastguard, descended on the ship, and initially were asked to extract 23 people from the Amsterdam-bound vessel, RAF Squadron Leader Dave Webster said.
But only 15 passengers needed to be checked for smoke-related injuries, and the smoke only affected six people -- four crew members and two passengers -- enough to merit airlifting them to a hospital, DFDS spokesman Gert Jakobsen said.
The fire broke out about four to five hours after the ship embarked from Newcastle, and the ferry's crew put it out quickly, Jakobsen said.
It's unclear whether the fire was accidental or intentional, but the ship's crew detained the passenger from the cabin where it occurred, Humberside police inspector Andrew Dixon said.
After the incident, the ship reversed course, back toward the UK, with about 120 crew members and 944 passengers, Jakobsen said. The ferry was expected to reach Newcastle early Sunday morning.
Northumbria police will take over the investigation from there, Dixon said.
CNN's Neda Farshbaf and Azadeh Ansari contributed to this report