A Self-Defense Force helicopter discharges water over a state-owned mountain forest in Namie, Fukushima Prefecture, on the afternoon of April 30, 2017. (Mainichi)

NAMIE, Fukushima -- A fire broke out in a mountain forest near the crippled Fukushima No. 1 Nuclear Power Plant on the evening of April 29, consuming an area approximately 20 hectares in size, according to prefectural authorities.

The fire started on 448-meter-high Mount Juman in Namie, Fukushima Prefecture, prompting the prefectural government to request the dispatch of the Ground Self-Defense Force (GSDF) on a disaster relief mission on April 30. A total of eight helicopters from Fukushima, Miyagi and Gunma prefectures as well as the SDF discharged water on the site to combat the fire.

As the fire continued to spread, however, helicopters from the GSDF, Fukushima Prefecture and other parties on May 1 resumed fire extinguishing operations from around 5 a.m.

The area is designated as a "difficult-to-return zone" due to high radiation levels from the Fukushima nuclear disaster, and entry into the area is barred in principle.

According to the Fukushima Meteorological Office, a lightning advisory had been issued for the town of Namie when the fire broke out, and Fukushima Prefectural Police suspect that lightning was to blame for the blaze as they continue to investigate the cause of the incident.

As of May 1, there were no major changes to radiation levels in the heart of Namie and other areas near the fire scene, according to the Ministry of the Environment.

"We will continue to closely watch changes in radiation doses in the surrounding areas," said a ministry official.