External power source had been connected to all the six reactors at the Fukushima Daiichi nuclear power plant, Kyodo News reported late Tuesday.

The No. 3 reactor was the last among six reactors to be connected to an external power supply, following the No. 4 reactor, which was connected in the morning.

According to the plant's operator, TEPCO, the reactors being set to receive power from independent sources is a critical first step towards the reactors' cooling systems working autonomously.

The utility has been battling to get the six reactors at the plant back online following the March 11 magnitude 9.0 mega quake and tsunami that knocked out the plant's critical cooling systems and led to partial meltdowns of some of the reactors' cores.