Official sources said there has been power transmission failure between Malda in West Bengal and Bongaigaon in Assam since this evening.
The power feed for the NE states is transmitted through that sector.
The problem was yet to be properly detected and the possibility of the fault not being restored by tonight had also not been ruled out.
The NE states had adopted a system of major load shedding and were supplying electricity on a rotational basis.
Power supply in Assam capital Guwahati and Meghalaya capital Shillong, among others, had been seriously affected since early evening with most parts of the cities in darkness.