The vehicle carrying 18 personnel from Madras Regiment Territorial Army based at Chapakhowa, Sadiya in Assam's Tinsukia district was on counter-insurgency operations.
The mishap occurred after their vehicle skidded and fell into Kundil river between Iduli and Kabang villages in the district, said sources in the army.
While 15 personnel were rescued from the river, four of the injured soldiers among them were shifted to the the Chapakhowa First Referral Unit for treatment.
District officials said search for the missing soldier is on and the National Disaster Response Force (NDRF) has been pressed into service.
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