The bus was on its way from state capital Aizawl to its southernmost Siaha district, they added.
Initial reports suggested that nine passengers had died in the accident while, six were admitted to the Serchhip hospital in a critical condition, police said.
Later, two passengers died because of their injuries raising the death toll to 11, they added.
Majority of the victims are from Siaha district, police said.
The bodies of the deceased would be sent to their respective villages for funeral after post-mortem examination, a police officer said.
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