Story highlights Five people remain missing and 12 are seriously injured, authorities say

The TGV was operating on a new stretch of line that's due to open next year

(CNN) At least 10 people were killed in eastern France on Saturday after a high-speed train came off the tracks and plunged into a canal during a test run, authorities said.

Around 50 people -- mostly engineers and other railway staff -- were aboard the TGV when it derailed outside the city of Strasbourg, according to the French railway operator SNCF.

Images from the scene showed sections of the train partially submerged in the canal, near a bridge.

Besides those killed, five other people remain unaccounted for, 12 were seriously injured and 22 were slightly injured, the French Ministry of Ecology, Sustainable Development and Energy said in a statement.

The news agency Agence France-Presse reported that it was the country's first fatal TGV accident in three decades.

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