Plant operator TEPCO said over the weekend that it had succeeded in bringing a small amount of debris outside the containment vessel surrounding a reactor -- using specialised robots able to function around the high radiation levels inside.

On Tuesday, Kuniaki Takahashi of TEPCO told reporters that they had measured the sample's radiation level and it was low enough to continue to the next stage of the complex process.

Around 880 tons of extremely hazardous material remain, more than 13 years after a catastrophic tsunami caused by a 9.0-magnitude earthquake triggered one of the world's worst nuclear accidents.

A trial removal of nuclear debris from the plant in northeastern Japan began in September, with the aim of studying a tiny sample for clues about conditions inside the reactors.

Removing the debris from the reactors is regarded as the most daunting challenge in the decades-long decommissioning project due to radiation levels within.

The company hopes to transport the sample to a research facility for further testing.

"We will move the debris into a box tomorrow for transportation... and then we will put it in a (special) container," Takahashi said.

"The retrieval will be complete after it is moved into the container," he said, adding that the timeframe for the next steps was unknown.

Three of Fukushima's six reactors went into meltdown after the huge tsunami swamped the facility.

Japan last year began releasing into the Pacific Ocean some of the 540 Olympic swimming pools' worth of reactor cooling water amassed since the disaster.

The step has sparked a diplomatic row with China and Russia, both of which banned seafood imports, although Japan insists the discharge is safe, a view backed by the UN atomic agency.

Beijing, however, said in September it would "gradually resume" importing seafood from Japan after imposing the blanket ban.