The gadgets were deployed in preparation for the removal of hundreds of tonnes of highly radioactive fuel and rubble, a risky operation expected to take decades.

"We sent two drones yesterday and two drones today", in addition to the "snake-shaped robot" on Thursday, a spokesman for Tokyo Electric Power (TEPCO) told AFP.

Since the interior is "cramped and dark", the small drones are "highly manoeuvrable and have enhanced photographic capabilities", the company said.

The "snake-like robot... houses wireless communications relay so that we can properly cover the radio transmission area within which the small drones will be operated", a statement said.

But later in the day, the probe was curtailed due to technical glitches, local media including public broadcaster NHK said.

The snake-like robot "could not reach" its destination because its cables did not function properly, with immediate resumption nowhere in sight, the Mainichi daily said.

The interior of the reactor buildings is too radioactive for people to enter, and the drones are meant to inspect the area before the removal of the fuel and rubble by robots.

TEPCO plans to carry out a trial removal of a small amount of fuel debris in October.

It had already sent an underwater robot to inspect parts of the facility still submerged, the spokesman said.

"It will take a long time to fully take off the 800 tonnes (of fuel), as the decommissioning period is thought to be 30 to 40 years," he said.

Separately, TEPCO on Wednesday began releasing a fourth batch of treated wastewater from the Fukushima plant into the ocean.

The process has been given the green light by the UN nuclear watchdog but China and Russia have banned seafood imports from Japan in response.

The 2011 earthquake and tsunami killed around 18,000 people. The catastrophe at the Fukushima nuclear power facility in northeast Japan was one of the worst atomic accidents in history.

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TEPCO - TOKYO ELECTRIC POWER