"We are aware of a letter from the anti-talk faction of ULFA threatening subversive activities tomorrow and hence a security alert has been sounded", Assam IGP (law and order) L R Bishnoi told PTI here today.
There is a general alert in upper Assam because of the fresh threat and a watch is being maintained in the all entry and exit point of major towns including Sibsagar, Jorhat, Dibrugarh and Tinsukia, he said.
Meanwhile, a hardcore ULFA ultra was killed in an encounter with the 12 Assam Rifles personnel at Namsai in the neighbouring state of Arunachal Pradesh, the police said here.
The identity of the militant was not yet known.
The ULFA's anti talk faction led by elusive commander in chief Paresh Baruah has threatened subversive activities till October 2.