The militants fired about 15 rounds on the vehicle of Mahalaxmi Tea Estate proprietor Adilur Rahman at Nagda on the Assam-Arunachal Pradesh border as he was driving with his security guard towards Baihata Chariali.
Rahman was killed and guard Motilal Tirke, belonging to the Assam Tea Protection Force, was seriously injured, they said.
Tirke was referred to the Gauhati Medical College Hospital where his condition was stated to be stable.
Following the incident, Assam Chief Minister Tarun Gogoi said operations against the anti-talk faction of the NDFB would be intensified.
"I have directed the security forces to take strong measures against the anti-talk faction of the NDFB so that we can crush the group," he told reporters in Guwahati.
He said an inquiry would be carried out into the killing of Rahman who had been warned by the police to be careful in his movements.
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"The DGP of state police has informed me that Rahman had been told to be careful about his movements as he was considered to be a target of the militants," he said.
Talking to television channels here, while on his way to the hospital, Tirke said around 12 armed men fired at their car from both sides of the road.
He sustained severe injuries in his hands.