HALIFAX -- ExxonMobil Canada says a helicopter on its way to an off-shore gas platform near Nova Scotia had to return to the Halifax International Airport.

The Sikorsky S-92 helicopter operated by Canadian Helicopter Offshore experienced an unexplained drop in altitude as it approached the Thebaud platform.

ExxonMobil says the two pilots decided to return to the airport with their 11 passengers.

No injuries were reported.

The company says Transport Canada Civil Aviation and the Transportation Safety Board have been notified.

ExxonMobil and the helicopter company continue to investigate, and passenger flights to offshore platforms have been suspended until the cause of the drop can be determined.