High-radiation zones contaminated by fallout from the 2011 Fukushima No. 1 atomic plant meltdown disaster had as of March shrunk to less than 25 percent of November 2011 levels, the Nuclear Regulation Authority said Wednesday.

About seven months after the start of the nuclear catastrophe, zones with radiation over 19 microsieverts per hour accounted for 27 percent of the total area designated as "difficult to return to for at least five years."

But the proportion of the zones had dropped to 16 percent as of last June 28, and to 6 percent as of March 11, according to aircraft monitoring surveys.

The government is expected to use the data in its planning to enable evacuees to return to their homes.

Since the Fukushima No. 1 complex achieved what was declared a stable state of cold shutdown in December 2011, the government has been reclassifying evacuation zones to places designated as "difficult-to-return-to" or "habitation-restricted."

The difficult-to-return-to zones, which are defined as areas with radiation over 50 millisieverts per year, currently total about 320 sq. km, excluding the 3-km radius from the plant.