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

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

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

Samples in the latest survey were collected last Tuesday from 11 points off the coast of Fukushima Prefecture, including a spot near the water discharge outlet and a location as far as about 50 kilometers away.

The ministry said the concentration of tritium at all the points was below the minimum detectable level of 10 becquerels per liter.

The tritium was also below the detectable level in the previous four surveys.

The ministry plans to continue monitoring tritium levels once a week for the time being. The results will be published 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.