Japan's Environment Ministry says tritium in seawater around the crippled Fukushima Daiichi nuclear power plant has remained below the detectable level.

The ministry announced the results of its third weekly survey of water samples on Monday.

Ministry officials have been conducting the survey since Tokyo Electric Power Company started releasing treated and diluted water from the Fukushima Daiichi nuclear power plant on August 24.

In the latest survey, samples were collected last Tuesday and Wednesday from 11 points, including a spot near the water discharge outlet and a location about 70 kilometers away.

It covered an area wider than the previous two surveys, spanning waters off Fukushima, Miyagi and Ibaraki prefectures.

The Environment Ministry says the concentration of tritium in all 11 points was under 10 becquerels per liter -- the minimum detectable level.

For the time being, the ministry plans to keep monitoring tritium levels once every week and releasing the results on its website and social media.

The Fukushima Daiichi plant suffered a triple meltdown in the 2011 earthquake and tsunami. Water used to cool molten fuel has been mixing with rain and groundwater.

The accumulated water is being treated to remove most radioactive substances, but still contains tritium.

Before releasing the treated water into the sea, the plant's operator dilutes it to reduce tritium levels to about one-seventh of the World Health Organization's guidelines for drinking water.