PARIS/WASHINGTON/MONTREAL (Reuters) - Boeing is investigating a new quality problem with its 787 Dreamliner after discovering that hundreds of fasteners have been incorrectly installed on the fuselages of some undelivered 787 Dreamliner jets, two people familiar with the matter said.

The latest in a series of manufacturing snags affecting the U.S. planemaker involves incorrect "torquing" or tightening in a Boeing plant of more than 900 fasteners per plane - split equally between both sides of the jet's mid-body, they said.

There is no immediate concern about flight safety but Boeing is working to understand what caused the problem and will decide how much if any rework needs to be done once its investigation is complete, the sources said, asking not to be identified.

The sources said the fasteners - which attach the carbon-composite skin to skeletal supports inside the fuselage called longerons - had been torqued from the wrong side.

Boeing confirmed the checks in response to a Reuters query.

"Our 787 team is checking fasteners in the side-of-body area of some undelivered 787 Dreamliner airplanes to ensure they meet our engineering specifications. The in-service fleet can continue to safely operate," a spokesperson said.

"We are taking the time necessary to ensure all airplanes meet our delivery standards prior to delivery. We are working closely with our customers and the FAA (Federal Aviation Administration) and keeping them updated."

The Federal Aviation Administration did not immediately respond to a request for comment.

Deliveries have been running more slowly than usual after an earlier production slowdown but have not so far been paused, FlightRadar24 tracking data showed.