In a statement e-mailed to PTI, the outfit's Publicity Secretary Arunodoy Dohotiya said the ULFA had always condemned attacks on indigenous Assamese and their properties.
"We condemn the attack on the Assamese people and will continue to do so in the future," he said.
Meanwhile, the North East Frontier Railway has confirmed that an improvised explosive device was used to trigger the blast that led to the derailment.
The NF railway has also filed an FIR with the government railway police against unidentified insurgents for carrying out the blast, Rangiya Divisional Railway Manager A K Manocha said. No militant outfit has so far claimed responsibility for the blast.